Visual space is the experience of space by an aware observer.
It is the subjective counterpart of the space of physical objects.
There is a long history in philosophy, and later psychology of writings
describing visual space, and its relationship to the space of physical
objects. A partial list would include René Descartes, Immanuel Kant, Hermann von Helmholtz, William James, to name just a few.
Object Space and Visual Space.
Space of physical objects
The
location and shape of physical objects can be accurately described with
the tools of geometry. For practical purposes the space we occupy is Euclidean. It is three-dimensional and measurable using tools such as rulers. It can be quantified using co-ordinate systems like the Cartesian x,y,z, or polar coordinates with angles of elevation, azimuth and distance from an arbitrary origin.
Space of visual percepts
Percepts,
the counterparts in the aware observer's conscious experience of
objects in physical space, constitute an ordered ensemble or, as Ernst Cassirer explained,
Visual Space can not be measured with rulers. Historically
philosophers used introspection and reasoning to describe it. With the
development of Psychophysics, beginning with Gustav Fechner,
there has been an effort to develop suitable experimental procedures
which allow objective descriptions of visual space, including geometric
descriptions, to be developed and tested. An example illustrates the
relationship between the concepts of object and visual space. Two
straight lines are presented to an observer who is asked to set them so
that they appear parallel. When this has been done, the lines are
parallel in visual space A comparison is then possible with the actual
measured layout of the lines in physical space. Good precision can be
achieved using these and other psychophysical procedures in human
observers or behavioral ones in trained animals.
Visual Space and the Visual Field
The visual field, the area or extent of physical space that is being imaged on the retina, should be distinguished from the perceptual space in which visual percepts are located, which we call visual space. Confusion is caused by the use of Sehraum in the German literature for both. There is no doubt that Ewald Hering and his followers meant visual space in their writings.
The fundamental distinction was made by Rudolf Carnap between three kinds of space which he called formal, physical and perceptual.
Mathematicians, for example, deal with ordered structures, ensembles of
elements for which rules of logico-deductive relationships hold,
limited solely by being not self-contradictory. These are the formal spaces. According to Carnap, studying physical
space means examining the relationship between empirically determined
objects. Finally, there is the realm of what students of Kant know as Anschauungen, immediate sensory experiences, often awkwardly translated as "apperceptions", which belong to perceptual spaces.
Visual space and geometry
Geometry
is the discipline devoted to the study of space and the rules relating
the elements to each other. For example, in Euclidean space the Pythagorean theorem provides a rule to compute distances from Cartesian coordinates.
In a two-dimensional space of constant curvature, like the surface of a
sphere, the rule is somewhat more complex but applies everywhere. On
the two-dimensional surface of a football, the rule is more complex
still and has different values depending on location. In well-behaved
spaces such rules used for measurement, called metrics, are classically handled by the mathematics invented by Riemann. Object space belongs to that class.
To the extent that it is reachable by scientifically acceptable
probes, visual space as defined is also a candidate for such
considerations. The first and remarkably prescient analysis was
published by Ernst Mach in 1901. Under the heading On Physiological as Distinguished from Geometrical Space
Mach states that "Both spaces are threefold manifoldnesses" but the
former is "...neither constituted everywhere and in all directions
alike, nor infinite in extent, nor unbounded." A notable attempt at a
rigorous formulation was made in 1947 by Rudolf Luneburg, who preceded his essay on mathematical analysis of vision
by a profound analysis of the underlying principles. When features are
sufficiently singular and distinct, there is no problem about a
correspondence between an individual item A in object space and its correlate A' in visual space. Questions can be asked and answered such as "If visual percepts A',B',C' are correlates of physical objects A,B,C, and if C lies between A and B, does C' lie between A' and B' ?"
In this manner, the possibility of visual space being metrical can be
approached. If the exercise is successful, a great deal can be said
about the nature of the mapping of the physical space on the visual
space.
On the basis of fragmentary psychophysical data of previous
generations, Luneburg concluded that visual space was hyperbolic with
constant curvature, meaning that elements can be moved throughout the
space without changing shape. One of Luneburg's major arguments is that,
in accord with a common observation, the transformation involving
hyperbolic space renders infinity into a dome (the sky). The Luneburg
proposition gave rise to discussions and attempts at corroborating
experiments, which on the whole did not favor it.
Basic to the problem, and underestimated by Luneburg the
mathematician, is the likely success of a mathematically viable
formulation of the relationship between objects in physical space and
percepts in visual space. Any scientific investigation of visual space
is colored by the kind of access we have to it, and the precision,
repeatability and generality of measurements. Insightful questions can
be asked about the mapping of visual space to object space
but answers are mostly limited in the range of their validity. If the
physical setting that satisfies the criterion of, say, apparent
parallelism varies from observer to observer, or from day to day, or
from context to context, so does the geometrical nature of, and hence
mathematical formulation for, visual space.
All these arguments notwithstanding, there is a major concordance
between the locations of items in object space and their correlates in
visual space. It is adequately veridical for us to navigate very
effectively in the world, deviations from such a situation are
sufficiently notable to warrant special consideration. visual space agnosia is a recognized neurological condition, and the many common distortions, called geometrical-optical illusions, are widely demonstrated but of minor consequence.
Neural representation of space
Fechner's inner and outer psychophysics
Its founder, Gustav Theodor Fechner defined the mission of the discipline of psychophysics
as the functional relationship between the mental and material
worlds—in this particular case, the visual and object spaces—but he
acknowledged an intermediate step, which has since blossomed into the
major enterprise of modern neuroscience. In distinguishing between inner and outer
psychophysics, Fechner recognized that a physical stimulus generates a
percept by way of an effect on the organism's sensory and nervous
systems. Hence, without denying that its essence is the arc between
object and percept, the inquiry can concern itself with the neural
substrate of visual space.
Retinotopy and beyond
The retina image topography is maintained through the visual pathway to the primary visual cortex.
Two major concepts dating back to the middle of the 19th century set the parameters of the discussion here. Johannes Müller emphasized that what matters in a neural path is the connection it makes, and Hermann Lotze, from psychological considerations, enunciated the principle of local sign.
Put together in modern neuroanatomical terms they mean that a nerve
fiber from a fixed retinal location instructs its target neurons in the
brain about the presence of a stimulus in the location in the eye's
visual field that is imaged there. The orderly array of retinal
locations is preserved in the passage from the retina to the brain, and
provides what is aptly called a "retinotopic" mapping in the primary visual cortex.
Thus in the first instance brain activity retains the relative spatial
ordering of the objects and lays the foundations for a neural substrate
of visual space.
Unfortunately simplicity and transparency ends here. Right at the
outset, visual signals are analyzed not only for their position, but
also, separately in parallel channels, for many other attributes such as
brightness, color, orientation, depth. No single neuron or even
neuronal center or circuit represents both the nature of a target
feature and its accurate location. The unitary mapping of object space
into the coherent visual space without internal contradictions or
inconsistencies that we as observer automatically experience, demands
concepts of conjoint activity in several parts of the nervous system
that is at present beyond the reach of neurophysiological research.
Though the details of the process by which the experience of visual
space emerges remain opaque, a startling finding gives hope for future
insights. Neural units have been demonstrated in the brain structure
called hippocampus that show activity only when the animal is in a specific place in its environment.
Space and its content
Only on an astronomical scale are physical space and its contents interdependent, This major proposition of the general theory of relativity is of no concern in vision. For us, distances in object space are independent of the nature of the objects.
But this is not so simple in visual space. At a minim an observer
judges the relative location of a few light points in an otherwise dark
visual field, a simplistic extension from object space that enabled
Luneburg to make some statements about the geometry of visual space. In a
more richly textured visual world, the various visual percepts carry
with them prior perceptual associations which often affect their
relative spatial disposition. Identical separations in physical space
can look quite different (are quite different in visual space)
depending on the features that demarcate them. This is particularly so
in the depth dimension because the apparatus by which values in the
third visual dimension are assigned is fundamentally different from that
for the height and width of objects.
Even in monocular vision, which physiologically has only two dimensions, cues of size, perspective, relative motion etc. are used to assign depth differences to percepts.
Looked at as a mathematical/geometrical problem, expanding a
2-dimensional object manifold into a 3-dimensional visual world is
"ill-posed," i.e., not capable of a rational solution, but is
accomplished quite effectively by the human observer.
The problem becomes less ill-posed when binocular vision allows actual determination of relative depth by stereoscopy, but its linkage to the evaluation of distance in the other two dimensions is uncertain (see: stereoscopic depth rendition).
Hence, the uncomplicated three-dimensional visual space of every-day
experience is the product of many perceptual and cognitive layers
superimposed on the physiological representation of the physical world
of objects.
Do
we see what is really there? The two areas of the image marked A and B,
and the rectangle connecting them, are all of the same shade: our eyes
automatically "correct" for the shadow of the cylinder.
The philosophy of perception is concerned with the nature of perceptual experience and the status of perceptual data, in particular how they relate to beliefs about, or knowledge of, the world. Any explicit account of perception requires a commitment to one of a variety of ontological or metaphysical views. Philosophers distinguish internalist accounts, which assume that perceptions of objects, and knowledge or beliefs about them, are aspects of an individual's mind, and externalist accounts, which state that they constitute real aspects of the world external to the individual. The position of naïve realism—the
'everyday' impression of physical objects constituting what is
perceived—is to some extent contradicted by the occurrence of perceptual
illusions and hallucinations and the relativity of perceptual experience as well as certain insights in science. Realist conceptions include phenomenalism and direct and indirect realism. Anti-realist conceptions include idealism and skepticism.
Recent philosophical work have expanded on the philosophical features
of perception by going beyond the single paradigm of vision (for
instance, by investigating the uniqueness of olfaction).
Categories of perception
We may categorize perception as internal or external.
Internal perception (proprioception)
tells us what is going on in our bodies; where our limbs are, whether
we are sitting or standing, whether we are depressed, hungry, tired and
so forth.
External or sensory perception (exteroception),
tells us about the world outside our bodies. Using our senses of sight,
hearing, touch, smell, and taste, we perceive colors, sounds, textures,
etc. of the world at large. There is a growing body of knowledge of the
mechanics of sensory processes in cognitive psychology.
Mixed internal and external perception (e.g., emotion and certain
moods) tells us about what is going on in our bodies and about the
perceived cause of our bodily perceptions.
The philosophy of perception is mainly concerned with exteroception.
An
object at some distance from an observer will reflect light in all
directions, some of which will fall upon the corneae of the eyes, where it will be focussed upon each retina,
forming an image. The disparity between the electrical output of these
two slightly different images is resolved either at the level of the lateral geniculate nucleus or in a part of the visual cortex
called 'V1'. The resolved data is further processed in the visual
cortex where some areas have specialised functions, for instance area V5
is involved in the modelling of motion and V4 in adding colour. The
resulting single image that subjects report as their experience is
called a 'percept'. Studies involving rapidly changing scenes show the
percept derives from numerous processes that involve time delays. Recent fMRI studies show that dreams, imaginings and perceptions of things such as faces
are accompanied by activity in many of the same areas of brain as are
involved with physical sight. Imagery that originates from the senses
and internally generated imagery may have a shared ontology at higher levels of cortical processing.
Sound is analyzed in term of pressure waves sensed by the cochlea
in the ear. Data from the eyes and ears is combined to form a 'bound'
percept. The problem of how this is produced, known as the binding problem.
Perception is analyzed as a cognitive process in which information processing
is used to transfer information into the mind where it is related to
other information. Some psychologists propose that this processing gives
rise to particular mental states (cognitivism) whilst others envisage a direct path back into the external world in the form of action (radical behaviourism). Behaviourists such as John B. Watson and B.F. Skinner have proposed that perception acts largely as a process between a stimulus and a response but have noted that Gilbert Ryle's "ghost in the machine
of the brain" still seems to exist. "The objection to inner states is
not that they do not exist, but that they are not relevant in a
functional analysis". This view, in which experience is thought to be an incidental by-product of information processing, is known as epiphenomenalism.
Contrary to the behaviouralist approach to understanding the elements of cognitive processes, gestalt psychology sought to understand their organization as a whole, studying perception as a process of figure and ground.
Philosophical accounts of perception
Important philosophical problems derive from the epistemology of perception—how we can gain knowledge via perception—such as the question of the nature of qualia. Within the biological study of perception naive realism is unusable. However, outside biology modified forms of naive realism are defended. Thomas Reid, the eighteenth-century founder of the Scottish School of Common Sense,
formulated the idea that sensation was composed of a set of data
transfers but also declared that there is still a direct connection
between perception and the world. This idea, called direct realism, has
again become popular in recent years with the rise of postmodernism.
The succession of data transfers involved in perception suggests that sense data are somehow available to a perceiving subject that is the substrate of the percept. Indirect realism, the view held by John Locke and Nicolas Malebranche, proposes that we can only be aware of mental representations
of objects. However, this may imply an infinite regress (a perceiver
within a perceiver within a perceiver...), though a finite regress is
perfectly possible.
It also assumes that perception is entirely due to data transfer and
information processing, an argument that can be avoided by proposing
that the percept does not depend wholly upon the transfer and
rearrangement of data. This still involves basic ontological issues of
the sort raised by Leibniz Locke, Hume, Whitehead and others, which remain outstanding particularly in relation to the binding problem,
the question of how different perceptions (e.g. color and contour in
vision) are "bound" to the same object when they are processed by
separate areas of the brain.
Indirect realism (representational views) provides an account of issues such as perceptual contents, qualia, dreams, imaginings, hallucinations, illusions, the resolution of binocular rivalry, the resolution of multistable perception,
the modelling of motion that allows us to watch TV, the sensations that
result from direct brain stimulation, the update of the mental image by
saccades of the eyes and the referral of events backwards in time.
Direct realists must either argue that these experiences do not occur or
else refuse to define them as perceptions.
Idealism holds that reality is limited to mental qualities while
skepticism challenges our ability to know anything outside our minds.
One of the most influential proponents of idealism was George Berkeley who maintained that everything was mind or dependent upon mind. Berkeley's idealism has two main strands, phenomenalism in which physical events are viewed as a special kind of mental event and subjective idealism. David Hume is probably the most influential proponent of skepticism.
A fourth theory of perception in opposition to naive realism, enactivism, attempts to find a middle path between direct realist and indirect realist theories, positing that cognition is a process of dynamic interplay between an organism's sensory-motor capabilities and the environment it brings forth.
Instead of seeing perception as a passive process determined entirely
by the features of an independently existing world, enactivism suggests
that organism and environment are structurally coupled and
co-determining. The theory was first formalized by Francisco Varela,
Evan Thompson, and Eleanor Rosch in "The Embodied Mind".
Spatial representation
An aspect of perception that is common to both realists and anti-realists is the idea of mental or perceptual space. David Hume
concluded that things appear extended because they have attributes of
colour and solidity. A popular modern philosophical view is that the
brain cannot contain images so our sense of space must be due to the
actual space occupied by physical things. However, as René Descartes
noticed, perceptual space has a projective geometry, things within it
appear as if they are viewed from a point. The phenomenon of perspective was closely studied by artists and architects in the Renaissance, who relied mainly on the 11th century polymath, Alhazen (Ibn al-Haytham), who affirmed the visibility of perceptual space in geometric structuring projections. Mathematicians now know of many types of projective geometry such as complex Minkowski space
that might describe the layout of things in perception (see Peters
(2000)) and it has also emerged that parts of the brain contain patterns
of electrical activity that correspond closely to the layout of the
retinal image (this is known as retinotopy). How or whether these become conscious experience is still unknown (see McGinn (1995)).
Beyond spatial representation
Traditionally,
the philosophical investigation of perception has focused on the sense
of vision as the paradigm of sensory perception.
However, studies on the other sensory modalities, such as the sense of
smell, can challenge what we consider characteristic or essential
features of perception. Take olfaction as an example. Spatial
representation relies on a "mapping" paradigm that maps the spatial
structures of the stimuli onto discrete neural structures and
representations.
However, olfactory science has shown us that perception is also a
matter of associative learning, observational refinement, and a
decision-making process that is context-dependent. One of the
consequences of these discoveries on the philosophy of perception is
that common perceptual effects such as conceptual imagery turn more on
the neural architecture and its development than the topology of the
stimulus itself.
The consciousness and binding problem is the problem of how objects, background and abstract or emotional features are combined into a single experience.
The binding problem refers to the overall encoding of our brain
circuits for the combination of decisions, actions, and perception. The
binding problem encompasses a wide range of different circuits and can
be divided into subsections that will be discussed later on. The binding
problem is considered a "problem" due to the fact that no complete
model exists.
The binding problem can be subdivided into four problems of perception, used in neuroscience, cognitive science and philosophy of mind. Including general considerations on coordination, the Subjective unity of perception, and variable binding.
General Considerations on Coordination
Summary of problem
Attention
is crucial in determining which phenomena appear to be bound together,
noticed, and remembered (Vroomen and Keetels, 2010). This specific
binding problem is generally referred to as temporal synchrony. At the
most basic level, all neural firing and its adaptation depends on
specific consideration to timing (Feldman, 2010). At a much larger
level, frequent patterns in large scale neural activity are a major
diagnostic and scientific tool.
Synchronization theory and research
A popular hypothesis mentioned by Peter Milner, in his 1974 article A Model for Visual Shape Recognition, has been that features of individual objects are bound/segregated via synchronization of the activity of different neurons in the cortex.
The theory, called binding-by-synchrony (BBS), is hypothesized to occur
through the transient mutual synchronization of neurons located in
different regions of the brain when the stimulus is presented.
Empirical testing of the idea was brought to light when von der
Malsburg proposed that feature binding posed a special problem that
could not be covered simply by cellular firing rates.
However, it has been shown this theory may not be a problem since it
was revealed that the modules code jointly for multiple features,
countering the feature-binding issue.
Temporal synchrony has been shown to be the most prevalent when
regarding the first problem, "General Considerations on Coordination,"
because it is an effective method to take in surroundings and is good
for grouping and segmentation. A number of studies suggested that there
is indeed a relationship between rhythmic synchronous firing and feature
binding. This rhythmic firing appears to be linked to intrinsic
oscillations in neuronal somatic potentials, typically in the gamma range around 40 - 60 hertz.
The positive arguments for a role for rhythmic synchrony in resolving
the segregational object-feature binding problem have been summarized by
Singer. There is certainly extensive evidence for synchronization of neural firing as part of responses to visual stimuli.
However, there is inconsistency between findings from different
laboratories. Moreover, a number of recent reviewers, including Shadlen
and Movshon and Merker
have raised concerns about the theory being potentially untenable.
Thiele and Stoner found that perceptual binding of two moving patterns
had no effect on synchronization of the neurons responding to two
patterns:coherent and noncoherent plaids.
In the primary visual cortex, Dong et al. found that whether two
neurons were responding to contours of the same shape or different
shapes had no effect on neural synchrony since synchrony is independent
of binding condition.
Shadlen and Movshon,
raise a series of doubts about both the theoretical and the empirical
basis for the idea of segregational binding by temporal synchrony. There
is no biophysical evidence that cortical neurons are selective to
synchronous input at this point of precision and cortical activity with
synchrony this precise is rare. Synchronization is also connected to
endorphin activity. It has been shown that precise spike timing may not
be necessary to illustrate a mechanism for visual binding and is only
prevalent in modeling certain neuronal interactions). In contrast, Seth
describes an artificial brain-based robot that demonstrates multiple,
separate, widely distributed neural circuits, firing at different
phases,showing that regular brain oscillations at specific frequencies
are essential to the neural mechanisms of binding).
Goldfarb and Treisman
point out that a logical problem appears to arise for binding solely
via synchrony if there are several objects that share some of their
features and not others. At best synchrony can facilitate segregation
supported by other means (as von der Malsburg acknowledges).
A number of neuropsychological studies suggest that the
association of color, shape and movement as "features of an object" is
not simply a matter of linking or "binding", but shown to be inefficient
to not bind elements into groups when considering association
and give extensive evidence for top-down feedback signals that ensure
that sensory data are handled as features of (sometimes wrongly)
postulated objects early in processing. Pylyshyn
has also emphasized the way the brain seems to pre-conceive objects
from which features are to be allocated to which are attributed
continuing existence even if features such as color change. This is
because visual integration increases over time and indexing visual
objects helps to ground visual concepts.
Feature integration theory
Summary of problem
The
visual feature binding problem refers to the question of why we do not
confuse a red circle and a blue square with a blue circle and a red
square. The understanding of the circuits in the brain stimulated for
visual feature binding is increasing. A binding process is required for
us to accurately encode various visual features in separate cortical
areas.
In her feature integration theory, Treisman suggested that one of
the first stages of binding between features is mediated by the
features' links to a common location. The second stage is combining
individual features of an object that requires attention, and selecting
that object occurs within a "master map" of locations. Psychophysical
demonstrations of binding failures under conditions of full attention
provide support for the idea that binding is accomplished through common
location tags.
An implication of these approaches is that sensory data such as
color or motion may not normally exist in "unallocated" form. For
Merker:
"The 'red' of a red ball does not float disembodied in an abstract
color space in V4." If color information allocated to a point in the
visual field is converted directly, via the instantiation of some form
of propositional logic (analogous to that used in computer design) into
color information allocated to an "object identity" postulated by a
top-down signal as suggested by Purves and Lotto (e.g. There is blue
here + Object 1 is here = Object 1 is blue) no special computational
task of "binding together" by means such as synchrony may exist.
(Although Von der Malsburg poses the problem in terms of binding "propositions" such as "triangle" and "top", these, in isolation, are not propositional.)
How signals in the brain come to have propositional content, or meaning, is a much larger issue. However, both Marr and Barlow
suggested, on the basis of what was known about neural connectivity in
the 1970s that the final integration of features into a percept would be
expected to resemble the way words operate in sentences.
The role of synchrony in segregational binding remains controversial. Merker
has recently suggested that synchrony may be a feature of areas of
activation in the brain that relates to an "infrastructural" feature of
the computational system analogous to increased oxygen demand indicated
via BOLD signal contrast imaging. Apparent specific correlations with
segregational tasks may be explainable on the basis of interconnectivity
of the areas involved. As a possible manifestation of a need to balance
excitation and inhibition over time it might be expected to be
associated with reciprocal re-entrant circuits as in the model of Seth
et al. (Merker gives the analogy of the whistle from an audio amplifier receiving its own output.)
Experimental Work
Visual
feature binding is suggested to have a selective attention to the
locations of the objects. If indeed spatial attention does play a role
in binding integration it will do so primarily when object location acts
as a binding cue. A study's findings has shown that functional MRI
images indicate regions of the parietal cortex involved in spatial
attention, engaged in feature conjunction tasks in single feature tasks.
The task involved multiple objects being shown simultaneously at
different locations which activated the parietal cortex. Whereas when
multiple objects are shown sequentially at the same location the
parietal cortex was less engaged.
Behavioral Experiments
Defoulzi
et al. Investigated feature binding through two feature dimensions, to
disambiguate whether a specific combination of color and motion
direction is perceived as bound or unbound. Two behaviorally relevant
features, including color and motion belonging to the same object, are
defined as the "bound" condition. Whereas the "unbound" condition has
features that belong to different objects. Local field potentials were
recorded from the lateral prefrontal cortex(lPFC) in monkeys and were
monitored during different stimulus configurations. The findings suggest
a neural representation of visual feature binding in 4 to 12 Hertz
frequency bands. It is also suggested that transmission of binding
information is relayed through different lPFC neural subpopulations. The
data shows a behavioral relevance of binding information that is linked
to the animal's reaction time. This includes the involvement of the
prefrontal cortex targeted by the dorsal and ventral visual streams in
binding visual features from different dimensions (color and motion).
It is suggested that the visual feature binding consists of two
different mechanisms in visual perception. One mechanism consists of
agonistic familiarity of possible combinations of features integrating
several temporal integration windows. It is speculated that this process
is mediated by neural synchronization processes and temporal
synchronization in the visual cortex. The second mechanism is mediated
by familiarity with the stimulus and is provided by attentional top-down
support from familiar objects.
Consciousness and Binding
Summary of Problem
Smythies
defines the combination problem, also known as the subjective unity of
perception, as "How do the brain mechanisms actually construct the
phenomenal object?". Revonsuo equates this to "consciousness-related binding", emphasizing the entailment of a phenomenal aspect. As Revonsuo explores in 2006,
there are nuances of difference beyond the basic BP1:BP2 division.
Smythies speaks of constructing a phenomenal object ("local unity" for
Revonsuo) but philosophers such as Descartes, Leibniz, Kant and James
(see Brook and Raymont)
have typically been concerned with the broader unity of a phenomenal
experience ("global unity" for Revonsuo) – which, as Bayne
illustrates may involve features as diverse as seeing a book, hearing a
tune and feeling an emotion. Further discussion will focus on this more
general problem of how sensory data that may have been segregated into,
for instance, "blue square" and "yellow circle" are to be re-combined
into a single phenomenal experience of a blue square next to a yellow
circle, plus all other features of their context. There is a wide range
of views on just how real this "unity" is, but the existence of medical
conditions in which it appears to be subjectively impaired, or at least
restricted, suggests that it is not entirely illusory.
There are many neurobiological theories about the subjective
unity of perception. Different visual features such as color, size,
shape, and motion are computed by largely distinct neural circuits but
we experience an integrated whole. The different visual features
interact with each other in various ways. For example, shape
discrimination of objects is strongly affected by orientation but only
slightly affected by object size. Some theories suggest that global perception of the integrated whole involves higher order visual areas. There is also evidence that the posterior parietal cortex is responsible for perceptual scene segmentation and organization.
Bodies facing each other are processed as a single unit and there is
increased coupling of the extrastriate body area (EBA) and the posterior
superior temporal sulcus (pSTS) when bodies are facing each other. This suggests that the brain is biased towards grouping humans in twos or dyads.
History
Early philosophers Descartes and Leibniz
noted that the apparent unity of our experience is an all-or-none
qualitative characteristic that does not appear to have an equivalent in
the known quantitative features, like proximity or cohesion, of
composite matter. William James
in the nineteenth century, considered the ways the unity of
consciousness might be explained by known physics and found no
satisfactory answer. He coined the term "combination problem", in the
specific context of a "mind-dust theory" in which it is proposed that a
full human conscious experience is built up from proto- or
micro-experiences in the way that matter is built up from atoms. James
claimed that such a theory was incoherent, since no causal physical
account could be given of how distributed proto-experiences would
"combine". He favoured instead a concept of "co-consciousness" in which
there is one "experience of A, B and C" rather than combined
experiences. A detailed discussion of subsequent philosophical positions
is given by Brook and Raymont (see 26). However, these do not generally
include physical interpretations.
Whitehead
proposed a fundamental ontological basis for a relation consistent with
James's idea of co-consciousness, in which many causal elements are
co-available or "compresent" in a single event or "occasion" that
constitutes a unified experience. Whitehead did not give physical
specifics but the idea of compresence is framed in terms of causal
convergence in a local interaction consistent with physics. Where
Whitehead goes beyond anything formally recognized in physics is in the
"chunking" of causal relations into complex but discrete "occasions".
Even if such occasions can be defined, Whitehead's approach still leaves
James's difficulty with finding a site, or sites, of causal convergence
that would make neurobiological sense for "co-consciousness". Sites of
signal convergence do clearly exist throughout the brain but there is a
concern to avoid re-inventing what Dennett calls a Cartesian Theater or single central site of convergence of the form that Descartes proposed.
Descartes's central "soul" is now rejected because neural
activity closely correlated with conscious perception is widely
distributed throughout the cortex. The remaining choices appear to be
either separate involvement of multiple distributed causally convergent
events or a model that does not tie a phenomenal experience to any
specific local physical event but rather to some overall "functional"
capacity. Whichever interpretation is taken, as Revonsuo
indicates, there is no consensus on what structural level we are
dealing with – whether the cellular level, that of cellular groups as
"nodes", "complexes" or "assemblies" or that of widely distributed
networks. There is probably only general agreement that it is not the
level of the whole brain, since there is evidence that signals in
certain primary sensory areas, such as the V1 region of the visual
cortex (in addition to motor areas and cerebellum), do not contribute
directly to phenomenal experience.
Experimental Work on the Biological Basis of Binding
fMRI work
Stoll
and colleagues conducted an fMRI experiment to see whether participants
would view a dynamic bistable stimulus globally or locally.
Responses in lower visual cortical regions were suppressed when
participants viewed the stimulus globally. However, if global perception
was without shape grouping, higher cortical regions were suppressed.
This experiment shows that higher order cortex is important in
perceptual grouping.
Grassi and colleagues used three different motion stimuli to
investigate scene segmentation or how meaningful entities are grouped
together and separated from other entities in a scene.
Across all stimuli, scene segmentation was associated with increased
activity in the posterior parietal cortex and decreased activity in
lower visual areas. This suggests that the posterior parietal cortex is
important for viewing an integrated whole.
EEG work
Mersad
and colleagues used an EEG frequency tagging technique to differentiate
between brain activity for the integrated whole object and brain
activity for parts of the object.
The results showed that the visual system binds two humans in close
proximity as part of an integrated whole. These results are consistent
with evolutionary theories that face-to-face bodies are one of the
earliest representations of social interaction. It also supports other experimental work showing that body-selective visual areas respond more strongly to facing bodies.
Electron tunneling
Experiments have shown that ferritin and neuromelanin in fixed human substantia nigra pars compacta (SNc) tissue are able to support widespread electron tunneling.
Further experiments have shown that ferritin structures similar to ones
found in SNc tissue are able to conduct electrons over distances as
great as 80 microns, and that they behave in accordance with Coulomb
blockade theory to perform a switching or routing function.
Both of these observations are consistent with earlier predictions that
are part of a hypothesis that ferritin and neuromelanin can provide a
binding mechanism associated with an action selection mechanism,
although the hypothesis itself has not yet been directly investigated.
The hypothesis and these observations have been applied to Integrated
Information Theory.
Modern theories
Dennett
has proposed that our sense that our experiences are single events is
illusory and that, instead, at any one time there are "multiple drafts"
of sensory patterns at multiple sites. Each would only cover a fragment
of what we think we experience. Arguably, Dennett is claiming that
consciousness is not unified and there is no phenomenal binding problem.
Most philosophers have difficulty with this position (see Bayne) but some physiologists agree with it. In particular, the demonstration of perceptual asynchrony in psychophysical experiments by Moutoussis and Zeki,
when color is perceived before orientation of lines and before motion
by 40 and 80 ms, respectively, constitutes an argument that, over these
very short time periods, different attributes are consciously perceived
at different times, leading to the view that at least over these brief
periods of time after visual stimulation, different events are not bound
to each other, leading to the view of a disunity of consciousness,
at least over these brief time intervals. Dennett's view might be in
keeping with evidence from recall experiments and change blindness
purporting to show that our experiences are much less rich than we sense
them to be – what has been called the Grand Illusion.
However, few, if any, other authors suggest the existence of multiple
partial "drafts". Moreover, also on the basis of recall experiments,
Lamme
has challenged the idea that richness is illusory, emphasizing that
phenomenal content cannot be equated with content to which there is
cognitive access.
Dennett does not tie drafts to biophysical events. Multiple sites
of causal convergence are invoked in specific biophysical terms by
Edwards and Sevush.
In this view the sensory signals to be combined in phenomenal
experience are available, in full, at each of multiple sites. To avoid
non-causal combination each site/event is placed within an individual
neuronal dendritic tree. The advantage is that "compresence" is invoked
just where convergence occurs neuro-anatomically. The disadvantage, as
for Dennett, is the counter-intuitive concept of multiple "copies" of
experience. The precise nature of an experiential event or "occasion",
even if local, also remains uncertain.
The majority of theoretical frameworks for the unified richness
of phenomenal experience adhere to the intuitive idea that experience
exists as a single copy, and draw on "functional" descriptions of
distributed networks of cells. Baars
has suggested that certain signals, encoding what we experience, enter a
"Global Workspace" within which they are "broadcast" to many sites in
the cortex for parallel processing. Dehaene, Changeux and colleagues have developed a detailed neuro-anatomical version of such a workspace. Tononi and colleagues
have suggested that the level of richness of an experience is
determined by the narrowest information interface "bottleneck" in the
largest sub-network or "complex" that acts as an integrated functional
unit. Lamme
has suggested that networks supporting reciprocal signaling rather than
those merely involved in feed-forward signaling support experience.
Edelman and colleagues have also emphasized the importance of re-entrant
signaling. Cleeremans emphasizes meta-representation as the functional signature of signals contributing to consciousness.
In general, such network-based theories are not explicitly
theories of how consciousness is unified, or "bound" but rather theories
of functional domains within which signals contribute to unified
conscious experience. A concern about functional domains is what
Rosenberg
has called the boundary problem; it is hard to find a unique account of
what is to be included and what excluded. Nevertheless, this is, if
anything is, the consensus approach.
Within the network context, a role for synchrony has been invoked
as a solution to the phenomenal binding problem as well as the
computational one. In his book, The Astonishing Hypothesis, Crick appears to be offering a solution to BP2 as much as BP1. Even von der Malsburg,
introduces detailed computational arguments about object feature
binding with remarks about a "psychological moment". The Singer group also appear to be interested as much in the role of synchrony in phenomenal awareness as in computational segregation.
The apparent incompatibility of using synchrony to both segregate
and unify might be explained by sequential roles. However, Merker
points out what appears to be a contradiction in attempts to solve the
subjective unity of perception in terms of a functional (effectively
meaning computational) rather than a local biophysical, domain, in the
context of synchrony.
Functional arguments for a role for synchrony are in fact underpinned by analysis of local biophysical events. However, Merker
points out that the explanatory work is done by the downstream
integration of synchronized signals in post-synaptic neurons: "It is,
however, by no means clear what is to be understood by 'binding by
synchrony' other than the threshold advantage conferred by synchrony at,
and only at, sites of axonal convergence onto single dendritic
trees..." In other words, although synchrony is proposed as a way of
explaining binding on a distributed, rather than a convergent, basis the
justification rests on what happens at convergence. Signals for two
features are proposed as bound by synchrony because synchrony effects
downstream convergent interaction. Any theory of phenomenal binding
based on this sort of computational function would seem to follow the
same principle. The phenomenality would entail convergence, if the
computational function does.
The assumption in many of the quoted models suggest that
computational and phenomenal events, at least at some point in the
sequence of events, parallel each other in some way. The difficulty
remains in identifying what that way might be. Merker's
analysis suggests that either (1) both computational and phenomenal
aspects of binding are determined by convergence of signals on neuronal
dendritic trees, or (2) that our intuitive ideas about the need for
"binding" in a "holding together" sense in both computational and
phenomenal contexts are misconceived. We may be looking for something
extra that is not needed. Merker, for instance, argues that the
homotopic connectivity of sensory pathways does the necessary work.
Cognitive Science and Binding
In modern connectionism cognitive neuroarchitectures are developed (e.g. “Oscillatory Networks”, “Integrated Connectionist/Symbolic (ICS) Cognitive Architecture”, “Holographic Reduced Representations (HRRs)”, “Neural Engineering Framework (NEF)”) that solve the binding problem by means of integrative synchronizationmechanisms
(e.g. the (phase-)synchronized “Binding-by-synchrony (BBS)” mechanism)
(1) in perceptual cognition ("low-level cognition"): This is the
neurocognitive performance of how an object or event that is perceived
(e.g., a visual object) is dynamically "bound together" from its
properties (e.g., shape, contour, texture, color, direction of motion)
as a mental representation, i.e., can be experienced in the mind as a unified "Gestalt" in terms of Gestalt psychology
("feature binding", feature linking"),
(2) and in language cognition ("high-level cognition"): This is the
neurocognitive performance of how a linguistic unit (e.g. a sentence) is
generated by relating semantic concepts and syntactic roles to each
other in a dynamic way so that one can generate systematic and
compositional symbol structures and propositions that are experienced as
complex mental representations in the mind ("variable binding").
In philosophy, physicalism is the metaphysical thesis that "everything is physical", that there is "nothing over and above" the physical, or that everything supervenes on the physical. Physicalism is a form of ontological monism—a "one substance" view of the nature of reality as opposed to a "two-substance" (dualism) or "many-substance" (pluralism) view. Both the definition of "physical" and the meaning of physicalism have been debated.
Physicalism is closely related to materialism. Physicalism grew out of materialism with advancements of the physical sciences
in explaining observed phenomena. The terms are often used
interchangeably, although they are sometimes distinguished, for example
on the basis of physics describing more than just matter (including
energy and physical law).
According to a 2009 survey, physicalism is the majority view among philosophers, but there remains significant opposition to physicalism. Neuroplasticity has been used as an argument in support of a non-physicalist view. The philosophical zombie argument is another attempt to challenge physicalism.
Alternatively, outside of philosophy, physicalism could also refer to the preference or viewpoint that physics should be considered the best and only way to render truth about the world or reality.
Definition of physicalism in philosophy
The word "physicalism" was introduced into philosophy in the 1930s by Otto Neurath and Rudolf Carnap.
The use of "physical" in physicalism is a philosophical concept
and can be distinguished from alternative definitions found in the
literature (e.g. Karl Popper defined a physical proposition to be one which can at least in theory be denied by observation).
A "physical property", in this context, may be a metaphysical or
logical combination of properties which are physical in the ordinary
sense. It is common to express the notion of "metaphysical or logical
combination of properties" using the notion of supervenience: A property A is said to supervene on a property B if any change in A necessarily implies a change in B.
Since any change in a combination of properties must consist of a
change in at least one component property, we see that the combination
does indeed supervene on the individual properties. The point of this
extension is that physicalists usually suppose the existence of various
abstract concepts which are non-physical in the ordinary sense of the
word; so physicalism cannot be defined in a way that denies the
existence of these abstractions. Also, physicalism defined in terms of
supervenience does not entail that all properties in the actual world
are type identical to physical properties. It is, therefore, compatible with multiple realizability.
From the notion of supervenience, we see that, assuming that
mental, social, and biological properties supervene on physical
properties, it follows that two hypothetical worlds cannot be identical
in their physical properties but differ in their mental, social or
biological properties.
Two common approaches to defining "physicalism" are the
theory-based and object-based approaches. The theory-based conception of
physicalism proposes that "a property is physical if and only if it
either is the sort of property that physical theory tells us about or
else is a property which metaphysically (or logically) supervenes on the
sort of property that physical theory tells us about".
Likewise, the object-based conception claims that "a property is
physical if and only if: it either is the sort of property required by a
complete account of the intrinsic nature of paradigmatic physical
objects and their constituents or else is a property which
metaphysically (or logically) supervenes on the sort of property
required by a complete account of the intrinsic nature of paradigmatic
physical objects and their constituents".
Physicalists have traditionally opted for a "theory-based" characterization of the physical either in terms of current physics, or a future (ideal) physics. These two theory-based conceptions of the physical represent both horns of Hempel's dilemma (named after the late philosopher of science and logical empiricist Carl Gustav Hempel):
an argument against theory-based understandings of the physical. Very
roughly, Hempel's dilemma is that if we define the physical by reference
to current physics, then physicalism is very likely to be false, as it
is very likely (by pessimistic meta-induction)
that much of current physics is false. But if we instead define the
physical in terms of a future (ideal) or completed physics, then
physicalism is hopelessly vague or indeterminate.
While the force of Hempel's dilemma against theory-based conceptions of the physical remains contested, alternative "non-theory-based" conceptions of the physical have also been proposed. Frank Jackson (1998) for example, has argued in favour of the aforementioned "object-based" conception of the physical. An objection to this proposal, which Jackson himself noted in 1998, is that if it turns out that panpsychism or panprotopsychism
is true, then such a non-materialist understanding of the physical
gives the counterintuitive result that physicalism is, nevertheless,
also true since such properties will figure in a complete account of
paradigmatic examples of the physical.
David Papineau and Barbara Montero have advanced and subsequently defended
a "via negativa" characterization of the physical. The gist of the via
negativa strategy is to understand the physical in terms of what it is
not: the mental. In other words, the via negativa strategy understands
the physical as "the non-mental". An objection to the via negativa
conception of the physical is that (like the object-based conception) it
doesn't have the resources to distinguish neutral monism (or panprotopsychism) from physicalism.
Further, Restrepo (2012) argues that this conception of the physical
makes core non-physical entities of non-´physicalist metaphysics, like
God, Cartesian souls and abstract numbers, physical and thus either
false or trivially true: "God is non-mentally-and-non-biologically
identifiable as the thing that created the universe. Sup- posing
emergentism is true, non-physical emergent properties are
non-mentally-and-non-biologically identifiable as non-linear effects of
certain arrangements of matter. The immaterial Cartesian soul is
non-mentally-and-non-biologically identifiable as one of the things that
interact causally with certain particles (coincident with the pineal
gland). The Platonic number eight is non-mentally-and-non-biologically
identifiable as the number of planets orbiting the Sun".
Supervenience-based definitions of physicalism
Adopting a supervenience-based account of the physical, the definition of physicalism as "all properties are physical" can be unraveled to:
1) Physicalism is true at a possible world w if and only if any world that is a physical duplicate of w is also a duplicate of w simpliciter.
Applied to the actual world (our world), statement 1 above is the
claim that physicalism is true at the actual world if and only if at every possible world
in which the physical properties and laws of the actual world are
instantiated, the non-physical (in the ordinary sense of the word)
properties of the actual world are instantiated as well. To borrow a
metaphor from Saul Kripke
(1972), the truth of physicalism at the actual world entails that once
God has instantiated or "fixed" the physical properties and laws of our
world, then God's work is done; the rest comes "automatically".
Unfortunately, statement 1 fails to capture even a necessary condition for physicalism to be true at a world w. To see this, imagine a world in which there are only
physical properties—if physicalism is true at any world it is true at
this one. But one can conceive physical duplicates of such a world that
are not also duplicates simpliciter of it: worlds that have the
same physical properties as our imagined one, but with some additional
property or properties. A world might contain "epiphenomenal ectoplasm",
some additional pure experience that does not interact with the
physical components of the world and is not necessitated by them (does
not supervene on them). To handle the epiphenomenal ectoplasm problem, statement 1 can be modified to include a "that's-all" or "totality" clause or be restricted to "positive" properties. Adopting the former suggestion here, we can reformulate statement 1 as follows:
2) Physicalism is true at a possible world w if and only if any world that is a minimal physical duplicate of w is a duplicate of w simpliciter.
Applied in the same way, statement 2 is the claim that physicalism is true at a possible world w if and only if any world that is a physical duplicate of w (without any further changes), is duplicate of w
without qualification. This allows a world in which there are only
physical properties to be counted as one at which physicalism is true,
since worlds in which there is some extra stuff are not "minimal"
physical duplicates of such a world, nor are they minimal physical
duplicates of worlds that contain some non-physical properties that are
metaphysically necessitated by the physical.
But while statement 2 overcomes the problem of worlds at which
there is some extra stuff (sometimes referred to as the "epiphenomenal
ectoplasm problem") it faces a different challenge: the so-called "blockers problem". Imagine a world where the relation between the physical and non-physical properties at this world (call the world w1)
is slightly weaker than metaphysical necessitation, such that a certain
kind of non-physical intervener—"a blocker"—could, were it to exist at w1, prevent the non-physical properties in w1 from being instantiated by the instantiation of the physical properties at w1. Since statement 2 rules out worlds which are physical duplicates of w1
that also contain non-physical interveners by virtue of the minimality,
or that's-all clause, statement 2 gives the (allegedly) incorrect
result that physicalism is true at w1. One response to
this problem is to abandon statement 2 in favour of the alternative
possibility mentioned earlier in which supervenience-based formulations
of physicalism are restricted to what David Chalmers
(1996) calls "positive properties". A positive property is one that
"...if instantiated in a world W, is also instantiated by the
corresponding individual in all worlds that contain W as a proper part." Following this suggestion, we can then formulate physicalism as follows:
3) Physicalism is true at a possible world w if and only if any world that is a physical duplicate of w is a positive duplicate of w.
On the face of it, statement 3 seems able to handle both the
epiphenomenal ectoplasm problem and the blockers problem. With regard to
the former, statement 3 gives the correct result that a purely physical
world is one at which physicalism is true, since worlds in which there
is some extra stuff are positive duplicates of a purely physical world.
With regard to the latter, statement 3 appears to have the consequence
that worlds in which there are blockers are worlds where positive
non-physical properties of w1 will be absent, hence w1 will not be counted as a world at which physicalism is true. Daniel Stoljar (2010) objects to this response to the blockers problem on the basis that since the non-physical properties of w1
aren't instantiated at a world in which there is a blocker, they are
not positive properties in Chalmers' (1996) sense, and so statement 3
will count w1 as a world at which physicalism is true after all.
A further problem for supervenience-based formulations of physicalism is the so-called "necessary beings problem".
A necessary being in this context is a non-physical being that exists
in all possible worlds (for example what theists refer to as God).
A necessary being is compatible with all the definitions provided,
because it is supervenient on everything; yet it is usually taken to
contradict the notion that everything is physical. So any
supervenience-based formulation of physicalism will at best state a necessary but not sufficient condition for the truth of physicalism.
Additional objections have been raised to the above definitions
provided for supervenience physicalism: one could imagine an alternate
world that differs only by the presence of a single ammonium molecule
(or physical property), and yet based on statement 1, such a world might
be completely different in terms of its distribution of mental
properties.
Furthermore, there are differences expressed concerning the modal
status of physicalism; whether it is a necessary truth, or is only true
in a world which conforms to certain conditions (i.e. those of
physicalism).
Realisation physicalism
Closely
related to supervenience physicalism, is realisation physicalism, the
thesis that every instantiated property is either physical or realised
by a physical property.
Token physicalism is the proposition that "for every actual
particular (object, event or process) x, there is some physical
particular y such that x = y". It is intended to capture the idea of
"physical mechanisms". Token physicalism is compatible with property dualism,
in which all substances are "physical", but physical objects may have
mental properties as well as physical properties. Token physicalism is
not however equivalent to supervenience physicalism. Firstly, token
physicalism does not imply supervenience physicalism because the former
does not rule out the possibility of non-supervenient properties
(provided that they are associated only with physical particulars).
Secondarily, supervenience physicalism does not imply token physicalism,
for the former allows supervenient objects (such as a "nation", or
"soul") that are not equal to any physical object.
Reductionism and emergentism
Reductionism
There are multiple versions of reductionism.
In the context of physicalism, the reductions referred to are of a
"linguistic" nature, allowing discussions of, say, mental phenomena to
be translated into discussions of physics. In one formulation, every
concept is analysed in terms of a physical concept. One counter-argument
to this supposes there may be an additional class of expressions which
is non-physical but which increases the expressive power of a theory.
Another version of reductionism is based on the requirement that one
theory (mental or physical) be logically derivable from a second.
The combination of reductionism and physicalism is usually called
reductive physicalism in the philosophy of mind. The opposite view is
non-reductive physicalism. Reductive physicalism is the view that mental
states are both nothing over and above physical states and reducible to
physical states. One version of reductive physicalism is type physicalism
or mind-body identity theory. Type physicalism asserts that "for every
actually instantiated property F, there is some physical property G such
that F=G". Unlike token physicalism, type physicalism entails supervenience physicalism.
Reductive versions of physicalism are increasingly unpopular as
they do not account for mental lives. The brain on this position as a
physical substance has only physical attributes such as a particular
volume, a particular mass, a particular density, a particular location, a
particular shape, and so on. However, the brain on this position does
not have any mental attributes. The brain is not overjoyed or unhappy.
The brain is not in pain. When a person's back aches and he or she is in
pain, it is not the brain that is suffering even though the brain is
associated with the neural circuitry that provides the experience of
pain. Reductive physicalism therefore cannot explain mental lives. In
the event of fear, for example, doubtlessly there is neural activity
that is corresponding with the experience of fear. However, the brain
itself is not fearful. Fear cannot be reduced to a physical brain state
even though it is corresponding with neural activity in the brain. For
this reason, reductive physicalism is argued to be indefensible as it
cannot be reconciled with mental experience.
Another common argument against type physicalism is multiple realizability,
the possibility that a psychological process (say) could be
instantiated by many different neurological processes (even
non-neurological processes, in the case of machine or alien
intelligence).
For in this case, the neurological terms translating a psychological
term must be disjunctions over the possible instantiations, and it is
argued that no physical law can use these disjunctions as terms.
Type physicalism was the original target of the multiple realizability
argument, and it is not clear that token physicalism is susceptible to
objections from multiple realizability.
There are two versions of emergentism, the strong version and the
weak version. Supervenience physicalism has been seen as a strong
version of emergentism, in which the subject's psychological experience
is considered genuinely novel.
Non-reductive physicalism, on the other side, is a weak version of
emergentism because it does not need that the subject's psychological
experience be novel. The strong version of emergentism is incompatible
with physicalism. Since there are novel mental states, mental states are
not nothing over and above physical states. However, the weak version
of emergentism is compatible with physicalism.
We can see that emergentism is actually a very broad view. Some
forms of emergentism appear either incompatible with physicalism or
equivalent to it (e.g. posteriori physicalism), others appear to merge both dualism
and supervenience. Emergentism compatible with dualism claims that
mental states and physical states are metaphysically distinct while
maintaining the supervenience of mental states on physical states. This
proposition however contradicts supervenience physicalism, which asserts
a denial of dualism.
A priori versus a posteriori physicalism
Physicalists
hold that physicalism is true. A natural question for physicalists,
then, is whether the truth of physicalism is deducible a priori
from the nature of the physical world (i.e., the inference is justified
independently of experience, even though the nature of the physical
world can itself only be determined through experience) or can only be
deduced a posteriori
(i.e., the justification of the inference itself is dependent upon
experience). So-called "a priori physicalists" hold that from knowledge
of the conjunction
of all physical truths, a totality or that's-all truth (to rule out
non-physical epiphenomena, and enforce the closure of the physical
world), and some primitive indexical truths such as "I am A" and "now is B", the truth of physicalism is knowable a priori.
Let "P" stand for the conjunction of all physical truths and laws, "T"
for a that's-all truth, "I" for the indexical "centering" truths, and
"N" for any [presumably non-physical] truth at the actual world. We can
then, using the material conditional "→", represent a priori physicalism as the thesis that PTI → N is knowable a priori. An important wrinkle here is that the concepts
in N must be possessed non-deferentially in order for PTI → N to be
knowable a priori. The suggestion, then, is that possession of the
concepts in the consequent, plus the empirical information in the antecedent is sufficient for the consequent to be knowable a priori.
An "a posteriori physicalist", on the other hand, will reject the
claim that PTI → N is knowable a priori. Rather, they would hold that
the inference from PTI to N is justified by metaphysical considerations
that in turn can be derived from experience. So the claim then is that
"PTI and not N" is metaphysically impossible.
One commonly issued challenge to a priori physicalism and to physicalism in general is the "conceivability argument", or zombie argument. At a rough approximation, the conceivability argument runs as follows:
P1) PTI and not Q (where "Q" stands for the conjunction of
all truths about consciousness, or some "generic" truth about someone
being "phenomenally" conscious [i.e., there is "something it is like" to be a person x] ) is conceivable (i.e., it is not knowable a priori that PTI and not Q is false).
P2) If PTI and not Q is conceivable, then PTI and not Q is metaphysically possible.
P3) If PTI and not Q is metaphysically possible then physicalism is false.
Here proposition P3 is a direct application of the
supervenience of consciousness, and hence of any supervenience-based
version of physicalism: If PTI and not Q is possible, there is some possible world
where it is true. This world differs from [the relevant indexing on]
our world, where PTIQ is true. But the other world is a minimal physical
duplicate of our world, because PT is true there. So there is a
possible world which is a minimal physical duplicate of our world, but
not a full duplicate; this contradicts the definition of physicalism
that we saw above.
Since a priori physicalists hold that PTI → N is a priori, they
are committed to denying P1) of the conceivability argument. The a
priori physicalist, then, must argue that PTI and not Q, on ideal
rational reflection, is incoherent or contradictory.
A posteriori physicalists, on the other hand, generally accept
P1) but deny P2)--the move from "conceivability to metaphysical
possibility". Some a posteriori physicalists think that unlike the
possession of most, if not all other empirical
concepts, the possession of consciousness has the special property that
the presence of PTI and the absence of consciousness will be
conceivable—even though, according to them, it is knowable a posteriori
that PTI and not Q is not metaphysically possible. These a posteriori
physicalists endorse some version of what Daniel Stoljar (2005) has
called "the phenomenal concept strategy".
Roughly speaking, the phenomenal concept strategy is a label for those a
posteriori physicalists who attempt to show that it is only the concept of consciousness—not the property—that is in some way "special" or sui generis. Other a posteriori physicalists
eschew the phenomenal concept strategy, and argue that even ordinary
macroscopic truths such as "water covers 60% of the earth's surface" are
not knowable a priori from PTI and a non-deferential grasp of the
concepts "water" and "earth" et cetera. If this is correct, then
we should (arguably) conclude that conceivability does not entail
metaphysical possibility, and P2) of the conceivability argument against
physicalism is false.
Other views
Realistic physicalism
Galen Strawson's realistic physicalism or realistic monism entails panpsychism – or at least micropsychism.
Strawson argues that "many—perhaps most—of those who call themselves
physicalists or materialists [are mistakenly] committed to the thesis
that physical stuff is, in itself, in its fundamental nature, something
wholly and utterly non-experiential... even when they are prepared to
admit with Eddington that physical stuff has, in itself, 'a nature
capable of manifesting itself as mental activity', i.e. as experience or
consciousness". Because experiential phenomena allegedly cannot be emergent from wholly non-experiential phenomena, philosophers are driven to substance dualism, property dualism, eliminative materialism and "all other crazy attempts at wholesale mental-to-non-mental reduction".
Real physicalists must accept that at least some ultimates are intrinsically experience-involving. They must at least embrace micropsychism.
Given that everything concrete is physical, and that everything
physical is constituted out of physical ultimates, and that experience
is part of concrete reality, it seems the only reasonable position, more
than just an 'inference to the best explanation'... Micropsychism is
not yet panpsychism, for as things stand realistic physicalists can
conjecture that only some types of ultimates are intrinsically
experiential. But they must allow that panpsychism may be true, and the
big step has already been taken with micropsychism, the admission that
at least some ultimates must be experiential. 'And were the inmost
essence of things laid open to us' I think that the idea that some but
not all physical ultimates are experiential would look like the idea
that some but not all physical ultimates are spatio-temporal (on the
assumption that spacetime is indeed a fundamental feature of reality). I
would bet a lot against there being such radical heterogeneity at the
very bottom of things. In fact (to disagree with my earlier self) it is
hard to see why this view would not count as a form of dualism... So now
I can say that physicalism, i.e. real physicalism, entails
panexperientialism or panpsychism. All physical stuff is energy, in one
form or another, and all energy, I trow, is an experience-involving
phenomenon. This sounded crazy to me for a long time, but I am quite
used to it, now that I know that there is no alternative short of
'substance dualism'... Real physicalism, realistic physicalism, entails
panpsychism, and whatever problems are raised by this fact are problems a
real physicalist must face.
— Galen Strawson, Consciousness and Its Place in Nature: Does Physicalism Entail Panpsychism?
