An introduction to
the philosophy of perception
Perception seen philosophically
• We perceive objects.
– Conceived metaphysically:
• What are the immediate objects of perception in
• What is there when you perceive an object?
– Conceived epistemologically:
• How do we gain knowledge of what is outside our
The Syllabus: Direct realism
• the immediate objects of perception are mind-
independent objects and their properties.
– Issues, including:
• the argument from illusion
• the argument from perceptual variation (Russell’s
• the argument from hallucination (the possibility
of experiences that are subjectively
indistinguishable from veridical perception)
• the time-lag argument.
The Syllabus: Indirect Realism
• the immediate objects of perception are mind-
dependent objects that are caused by and represent
mind- independent objects.
• Issues, including:
– it leads to scepticism about the ‘existence’ of the external world (attacking
‘realism’) – responses to this (external world is the ‘best hypothesis’
(Russell); coherence of the various senses and lack of choice over our
– it leads to scepticism about the ‘nature’ of the external world (attacking
‘representative’) - responses (sense data tell us of ‘relations’ between
objects (Russell); the distinction between primary and secondary qualities
– problems arising from the view that mind-dependent objects represent
mind-independent objects and are caused by mind-independent objects.
The Syllabus: Berkeley’s Idealism
• the immediate objects of perception (i.e. ordinary objects such
as tables, chairs, etc) are mind-dependent objects.
• Berkeley’s attack on the primary/secondary property distinction
and his ‘master’ argument.
• Issues, including:
– it leads to solipsism
– it does not give an adequate account of illusions and
– it cannot secure objective space and time
– whether God can be used to play the role He does.
Realism and Idealism in perception
• Perceptual realists, of whatever flavour, are
committed to the following:
– There are physical objects which are mind-
– These physical objects are
• Perceptual idealists, of whatever flavour, argue:
– that physical objects are mind-dependent.
– Physical objects are mental things in some way.
Theories of Perception, outlined
• Naïve or Direct Realism
– Physical objects exist and their qualities are perceived
directly by the perceiver.
• Indirect Realism
– Physical objects exist and their underlying physical
qualities generate perceptions in the mind of the
– Physical objects are simply perceptions in the mind of
Direct or Naïve Realism
• Direct Realists: Thomas Reid
• Direct realism:
– the immediate objects of perception are mind-independent
objects and their properties
– which are perceived directly by the perceiver
– so there are no intermediaries between object and perceiver
– Question: ‘What do I perceive?’ ‘I directly perceive mind-
independent physical objects and their properties’.
Thomas Reid: Roses
‘What is smell in the rose? It is a quality or virtue of the rose, or of something
given off by the rose, which we perceive through the sense of smelling… What
is smelling? It is an act of the mind, but is never imagined to be a quality of the
mind. Again, the sensation of smelling is conceived to imply necessarily a mind
or sentient being; but smell in the rose implies no such thing...So smell in the
rose and the sensation that it causes are not thought of, even by the vulgar, as
things of the same kind, although they have the same name.
From what I have said we can learn that `the smell of a rose' signifies two
• A sensation, which can't exist except when it is perceived, and can exist only
in a sentient being or mind.
• Some power, quality or virtue in the rose, or in effluvia that it gives off,
which has a permanent existence independently of the mind and which by
the constitution of nature produces the sensation in us.
We are fundamentally so built that we are led to believe that there is a
permanent cause of the sensation, and are prompted to look for it; and
experience leads us to locate it in the rose.
Direct Realism in Standard Form
(nicked from University of Reading Philosophy dept.)
John Hospers, in ‘Philosophical Analysis’. He also suggests that these beliefs
are shared “by virtually all human beings”.
1. There exists a world of material objects.
2. Statements about these objects can be known to be true through sense-
3. These objects exist not only when they are being perceived but also
when they are not perceived: the objects of perception are largely
4. These objects are also able to retain properties of the types we perceive
them as having even when they are not being perceived. Their
properties are perception-independent.
5. By means of our senses, we perceive the world directly, and pretty much
as it is.
6. So in the main, our claims to have knowledge of it are justified.
Four issues with Direct Realism
1. the argument from perceptual variation
(Russell’s table example)
2. the argument from illusion (given, for
example, by A.J. Ayer)
3. the argument from hallucination (the
possibility of experiences that are
subjectively indistinguishable from veridical
4. the time-lag argument (Russell again).
The argument from perceptual
variation: Russell’s table example
• To make our difficulties plain, let us concentrate attention on the table…as
soon as we try to be more precise our troubles begin. Although I believe
that the table is 'really' of the same colour all over, the parts that reflect the
light look much brighter than the other parts, and some parts look white
because of reflected light. I know that, if I move, the parts that reflect the
light will be different, so that the apparent distribution of colours on the
table will change…if several people are looking at the table at the same
moment, no two of them will see exactly the same distribution of
colours…It is evident from what we have found, that there is no colour
which pre-eminently appears to be the colour of the table, or even of any
one particular part of the table--it appears to be of different colours from
different points of view, and there is no reason for regarding some of these
as more really its colour than others…to avoid favouritism, we are
compelled to deny that, in itself, the table has anyone particular colour.
• Russell then applies the same argument to the other senses which we
might use to perceive the table.
Further developments of the perceptual
Further developments of the perceptual
The Perceptual Variation Argument in
1. Perceptions of the same object vary.
2. These variations occur without changes in the
3. So the properties physical objects really have
and those they appear to have are different.
4. So what we are directly aware of in perception is
not exactly the same as what exists
independently of our minds.
5. Therefore we do not perceive physical objects
So, Mr Russell, what are we directly
aware of in perception?
• ‘Let us give the name of 'sense-data' to the things that are immediately
known in sensation: such things as colours, sounds, smells, hardnesses,
roughnesses, and so on. We shall give the name 'sensation' to the
experience of being immediately aware of these things. Thus, whenever
we see a colour, we have a sensation of the colour, but the colour itself is a
sense-datum, not a sensation. The colour is that of which we are
immediately aware, and the awareness itself is the sensation. It is plain
that if we are to know anything about the table, it must be by means of
the sense-data--brown colour, oblong shape, smoothness, etc. – which we
associate with the table; but, for the reasons which have been given, we
cannot say that the table is the sense-data, or even that the sense-data
are directly properties of the table. Thus a problem arises as to the
relation of the sense-data to the real table, supposing there is such a
thing.’ (Russell, ‘Problems of Philosophy’, Chapter 1)
• In other words, Russell uses the perceptual variation problem as an
argument for indirect realism, by (re-)introducing the notion of sense-
data which 1) are caused by physical objects; 2) and which cause our
sensations in turn.
Responses the direct realist might
make: relational properties
• This objection assumes that if something appears E to subject S, then S
can only be immediately aware of something that in fact is E.
– e.g. if a circular table looks elliptical from one angle, then it must be elliptical
• But the direct realist does not have to hold this view. (Bertrand Russell is
setting up a straw man argument).
• Instead the direct realist can hold that external physical objects or events
may appear other than they are, using the notion of relational properties
i.e. a property an object has in relation to being perceived.
– e.g. ‘to the left of’ is a relational property that depends on the relation
between a perceiver and an object.
– ‘to the left of’ is a real property relating real objects. But it does vary between
• So a circular object may have the relational property of ‘appearing
elliptical’ to us from a certain angle of view – which is not at all the same
property as ‘being elliptical’.
– e.g. it could look elliptical but be circular.
The Direct Realist’s Response,
• And direct realists can account for such properties by straightforwardly
appealing to various physical and physiological considerations:
– e.g. a circular table may look elliptical (or of another size) from a certain angle
because of perspectival distortion;
– e.g. a copper penny of a certain hue may look to be of a different hue from
another angle for many reasons (lighting effects, the visual acuity of the
• There is nothing in such commonplace facts which means we must posit
the existence of something other than physical objects as the objects of
• And the notion of an ‘ideal observer’ seeing an object under normal
lighting conditions whilst in a normal frame of mind etc and so knowing
the real properties of the object is a perfectly defensible idea: this is what
we mean when we say e.g. ‘the table is circular, and blue’ etc
• That is: we perceive physical objects, not sense-data, but not all
properties of physical objects are mind-independent. (But! Is this the
primary/secondary quality distinction here?)
Appearance and Reality: the arguments from
illusion and hallucination
• These arguments should
– What are they?
– What’s the difference
between an illusion and a
Ayer’s argument from illusion
1. We perceive something having some property F e.g. a straight stick half
submerged in water which hence looks bent.
2. When we perceive something having some property F then there is
something that has this property.
3. But in the case of a perceptual illusion then the property F is not really
possessed by the object e.g. the stick is not really bent.
4. So the property F must be a mental construct (a sense-datum or idea).
(We only know the stick is really straight by inference from previous
5. In perceptual illusions, we perceive sense-data and not physical objects
6. But illusions can be subjectively indistinguishable from veridical
7. So in the case of both illusions and veridical perception we actually
perceive only sense-data.
8. So direct realism is false.
Countering the argument from illusion
• The argument involves a question-begging assumption that if something
appears F to subject S, then S is immediately aware of something that is F.
• But Direct Realists can hold that object O may appear F to S, even though
O is not F.
– As with the argument from perceptual variation, the object may have
relational properties as well as directly perceived physical ones.
– e.g. the stick may have the property of ‘looking bent when partly immersed in
water’ – and ‘looking bent’ and ‘being bent’ are different properties.
• In addition, there are perfectly good scientific explanations of illusions
which don’t involve positing other objects of awareness beyond the
– e.g. a straight stick submerged in water may look bent as water has a different
refractive index to air and so bends light differently. The straight stick's
appearing bent is thus explainable without needing to posit some tertium
quid which is bent.
the argument from hallucination
• What are hallucinations?
– ‘prolonged experiences that are subjectively
indistinguishable from veridical perception’
– Or: ‘perceptual illusions extended to cases where we
think we perceive things which in fact aren’t there at
all (rather than just misperceiving the properties of
things which are there to be perceived)’
– This kind of perceptual delusion is a more radical
kind of perceptual error than simple illusion.
• Again, why would they be a problem for the
The argument from hallucination in
1. Consider the proverbial hallucinator. He is clearly immediately
aware of something. For example, James Steward’s drunkard
in the 1950 movie ‘Harvey’ consistently hallucinates
interactions with a 6’ 8” rabbit.
2. But the object hallucinated does not actually exist. There is
no Harvey the rabbit.
3. So the hallucinator can only be immediately aware of
something other than an external physical object.
4. But there is no significant qualitative or phenomenal
difference between the objects of awareness in cases of
hallucination and in cases of veridical perception. Harvey and
an actual six foot tall rabbit may be phenomenally
indistinguishable to James Steward.
5. So we have reason to suppose that, since the objects of
immediate awareness in hallucination are not external
physical objects, the objects of immediate awareness in
veridical perception are also not external physical objects.
6. So Direct Realism is false and the objects of direct awareness
must be sense data and the like.
Countering the Argument from Hallucination
• The Argument from Hallucination may very well be the
most powerful argument against Direct Realism, but it fails
to refute it.
• Firstly even if we suppose for the sake of argument that
sense-data (or ideas or the like) are the objects of
immediate awareness in cases of hallucination, we need
not accept that they are also the objects of immediate
awareness in (veridical) perception. The proponent of the
argument from hallucination assumes that if x and y are
phenomenally indistinguishable, x and y are ontologically
indistinguishable. If it walks like a duck, quacks like a duck,
then it’s a duck.
• But counter-examples to this principle abound. A papier-
mache barn might appear phenomenally indistinguishable
from a real one; a fake Rolex might be physically incredibly
similar to a real one.
• Second, let's allow James Stewart his direct
awareness of Harvey. Even if a) Harvey doesn’t exist;
and b) yet James Stewart is aware of something, we
don’t have to conclude c) that he must be
experiencing only sense-data or the like.
• There could be other perfectly good and perfectly
scientific explanations of Harvey the rabbit. For
– In the special case of hallucinations, brain-states are the objects
of immediate awareness OR
– In the special case of hallucinations, the objects of awareness
are mental images.
Countering the Argument from Hallucination
the time-lag argument
given by Russell in ‘The Problems of Philosophy’
1. We cannot perceive physical objects or events unless light is reflected or
emitted by them and reaches our visual system.
2. Light travels at a finite velocity, and so there is always some time interval
between the reflection or emission of light from a physical object or event
and the light's reaching our eyes. Usually this interval is infinitesimal,
because objects are usually close to us. But in the case of a distant star, or
our sun, the time interval may be so considerable that, by the time the
light reaches our eyes, the star may no longer exist.
3. If something no longer exists, we cannot now perceive it, let alone
directly perceive it. And so, assuming the distant star no longer exists, we
cannot directly perceive it when its light reaches our eyes. But since we
are perceiving something, the object of (direct) perception must be
something other than the distant star.
4. Though time lags are most significant in cases of distant objects such as
stars, any time lag, however minute, between physical objects or events
and our perception of them is incompatible with Direct Realism, for given
the time lag, we cannot directly perceive physical objects and events as
they presently are at the time of our perception. Since we perceive
something, the object of (direct) perception must be something other
than physical objects or events.
5. Direct Realism is false. We do not directly perceive physical objects and
events. Enter sense-data, ideas etc.
Polaris, the North Star. It lies
along Earth’s axis of rotation so
appears not to move in the night
sky. It is 234 light-years or 234
trillion kilometres away. Is it still
Countering the time-lag argument
• Direct Realists can acknowledge the modern science of
• in the vast majority of cases of perception of nearby physical objects
or events, perception occurs so quickly that it seems to occur
• But all perception involves a time lag, however short;
• Astronomical perception simply has a more visible time lag because of
the distances involved.
• So awareness of a celestial body which no longer exists is not odd. It’s
• Hence Direct Realists can concede the first two premises, and
need only focus on the third and fourth premises, the crux of
Countering the time-lag argument
1. A confusion in the time-lag argument arises because the claim in
premise 3 ("if something no longer exists, we cannot now perceive it“)
can be interpreted in at least two distinct ways:
1. if something no longer exists, we cannot now perceive it as it presently is. True.
2. if something no longer exists, we cannot now perceive it as it used to be.
Questionable. Surely we can now be aware of something as it was?
2. Scientific Direct Realists don’t make the claim that we can be aware of
the no-longer existent object as it is now, but only that we can be aware
of the once-existent object as it used to be.
3. So: from the physical fact that there are time lags in perception
1. It does not follow that we cannot directly perceive external physical objects or events.
2. It only follows that we cannot directly perceive physical objects without a time lag.
3. But no scientific direct realist suggests human perception takes place instantaneously
(of course, we may feel that perception is instantaneous).
4. So direct realism is not defeated by the time-lag argument.
Notas do Editor
Assign different paragraphs of Russell’s arguments to the class and get them to present back.
Jaundice. Colour blindness (red/green, simulated), age-related hearing loss
Ask why these pictures are relevant. (Top left: bee vision simulation, r.h. picture being human optical wavelengths, left being bee), bat echolocation, snake heat vision (central ‘pit’ is IR detector), hammerhead shark as detector of electrical currents). All of these animals arguably see the world in very different ways from us.
Could show first arrival of Johnny Depp at his hotel in ‘Fear and Loathing in Las Vegas’, in which he hallucinates horribly. https://www.youtube.com/watch?v=bKECvqSVWPA is a James Stewart clip from Harvey. https://www.youtube.com/watch?v=Me_BI-dledQ clip from Donnie Darko.
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