The Big Picture
Part One: The Nature of Knowledge Weeks 2-3: What is Knowledge?
• A: Its somehow related to justification…
Week 4: What is Justification?
• A: It requires epistemically basic beliefs?
Part Two: Ways of Knowing Weeks 5-7: Perception
• Means of acquiring empirical knowledge
Weeks 8-9: A Priori Reasoning
• Means of acquiring non-empirical knowledge
7. Overview
• The Argument from Illusion
• Illusions
• Responses
The Argument from Illusion
1) Physical objects sometimes appear other than they really are
• Physical object is F, but appears G
• E.g. a tilted penny is round, but appears ellipical
The Argument from Illusion
2) When something appears to have a certain property, there is something that the subject is aware of—a
sense-datum—that really has that property (‘The Phenomenal Principle’)
• E.g. if a tilted penny appears elliptical, then there is something that really is elliptical
• Plausibly, perceiving (being aware of, etc.) is a relation, and relations have relata: perceiver and perceived.
• If so, ‘perceives’ a ‘success term’ (cf. ‘knows’, week 1).
• (Note: singular ‘sense-datum’, plural ‘sense-data’.)
The Argument from Illusion
3) Sense-data are distinct from physical objects
• Physical object is F, sense-datum is G
• E.g. penny is round, sense-datum is elliptical
• Leibniz’s Law: if a and b are identical, then everything that is true of a is true of b
• a = b ⊃ (∀F)(Fa ↔ Fb)
4) Therefore, in cases of illusion we are not
aware of physical objects, but sense-data
The Argument from Illusion
Generalising:
5) Illusory perceptual experiences are at least in principle indistinguishable from other types of perceptual experience
6) If illusory perceptual experiences are
indistinguishable from other types of perceptual experience, then these experiences are of the same fundamental kind
7) Therefore, we are never (directly) aware of physical
objects, but only sense-data
‘Subjective’ Illusions
• Blurred vision: remove glasses, get drunk
• Double Vision: move fingers close to eyes, gently(!) press side of eyeball
• Colour-blindness
• Jaundice(?)
‘Subjective’ Illusions
After-Images
‘Subjective’ Illusions(?)
Neon Colour Spreading
‘Environmental’ Illusions
The Muller-Lyer Illusion
‘Environmental’ Illusions
The Muller-Lyer Illusion
‘Environmental’ Illusions
Bent Stick in Water
‘Environmental’ Illusions
Shape Constancy Illusion
‘Environmental’ Illusions
Size Constancy Illusion
‘Environmental’ Illusions
Size Constancy Illusion
‘Environmental’ Illusions
Colour Constancy Illusion
Part of white square
in shadow appears
different to part of
white square that is
directly illuminated
‘Environmental’ Illusions
Colour Constancy Illusion
Part of white square in shadow appears different to part of white square that is directly illuminated Note also, that
square A is the same colour as square B!
http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html
‘Environmental’ Illusions
Lotto-Purves Cube
http://www.purveslab.net/seeforyourself/index.html?1.00
Hallucinations
• Drugs
• Evil Scientist
• Dreams?
• Note: the ‘Argument from Hallucination’ is slightly different to the ‘Argument from Illusion’. Can you
think how it might go?
The Argument from Illusion
1) Physical objects sometimes appear other than they really are 2) When something appears to have a certain property, there is
something that the subject is aware of—a sense-datum—that really has that property (‘The Phenomenal Principle’)
3) Sense-data are distinct from physical objects
4) Therefore, in cases of illusion we are not aware of physical objects, but sense-data
5) Illusory perceptual experiences are at least in principle
indistinguishable from other types of perceptual experience 6) If illusory perceptual experiences are indistinguishable from
other types of perceptual experience, then these experiences are of the same fundamental kind
7) Therefore, we are never aware of physical objects, but only sense-data
Responses: Premiss 1
1) Physical objects sometimes appear other than they really are
• In some sense, tilted pennies do not appear elliptical:
they appear to be round pennies tilted in space.
• ‘Constant’ properties (e.g. shape, size, colour) are:
– Phenomenologically salient – Epistemologically important
• Constancy depends on awareness of contextual cues:
orientation (shape), distance (size), illumination
(colour)
Responses: Premiss 1
Some Questions
• Do tilted pennies ‘appear’ round in the ‘phenomenal’
sense of ‘appears’?
– Do we literally perceive tilted pennies to be round?
– Or do we merely believe that they are round?
• Despite constancy, there is some difference between perceiving a penny face on and tilted in space. What?
• Constancy fails if contextual cues are removed (cf.
monsters in the tunnel)
• What about ‘subjective’ illusions and hallucinations?
Responses: Premiss 2
2) When something appears to have a certain property, there is something that the subject is aware of—a
sense-datum—that really has that property (‘The Phenomenal Principle’)
• Premiss rejected by ‘intentionalists’ (week 6).
• Perceptual experience is representational: it does not imply the existence of what it is a representation of
• Compare belief: I believe that Father Christmas is fat
does not imply that Father Christmas exists
Responses: Premiss 5
5) Illusory perceptual experiences are at least in principle indistinguishable from other types of perceptual experience
• Austin (p. 80): ‘I may have the experience (dubbed
“delusive” presumably) of dreaming that I am being presented to the Pope. Could it seriously be suggested that having this dream is “qualitatively
indistinguishable” from actually being presented to the Pope?’
• Cf. Berkeley: dreams are less coherent, and have less
‘force and vivacity’ than waking experiences
(Principles of Human Knowledge, §§30, 33, 41)
Responses: Premiss 5
• But are illusory perceptual experiences at least in principle indistinguishable from other types of perceptual experience?
• Two types of experience can have the same proximate cause in the brain. Plausibly, two experiences with
the same proximate cause could be indistinguishable.
Responses: Premiss 6
6) If illusory perceptual experiences are
indistinguishable from other types of perceptual experience, then these experiences are of the same fundamental kind
• Austin (p. 81): ‘If I am told that a lemon is generically different from a piece of soap, do I “expect” that no piece of soap could look just like a lemon? Why
should I?’
• Premiss rejected by ‘disjunctivists’ (week 7)
• Illusion/hallucination and veridical perception are
fundamentally different: no ‘highest common factor’
Responses: Accept?
7) Therefore, we are never aware of physical objects, but only sense-data
• If we accept the conclusion, then we are committed to
the sense-datum theory of perception. See lecture 8…
References
Online Illusions
http://www.purveslab.net/seeforyourself/index.html?1.00
http://www.gla.ac.uk/departments/philosophy/cspe/illusions/
The Argument from Illusion
T. Crane, ‘The Problem of Perception’, The Stanford Encyclopedia of Philosophy, http://plato.stanford.edu/entries/perception- problem/
H. Robinson, Perception (London: Routledge, 1994)
H.H. Price, Perception (London: Methuen, 1932), Chapter 1
