Perception as relational; episodes of perceptual awareness as relations of a subject of experience to something.
One central question in the philosophy of perception is: what are the objects of perception? Of what things does perception make us immediately aware?
Two answers to this question about the objects of perception may come to mind. One might think that we are made immediately aware (in the standard case) of material objects in the world: desks, chairs, etc. Call this the direct realist theory of perception. But one might also think that we get access to such material objects only indirectly, via immediate awareness of mental items. Call this latter view the sense datum theory of perception. (Some versions of this theory are also known as kinds of indirect realism.)
The argument from illusion is the most prominent argument in favor of the sense datum theory of perception; one of the most important presentations of the argument was in A. J. Ayer’s The Foundations of Empirical Knowledge. As Ayer puts it, the argument from illusion is his answer to the question, ‘Why may we not say that we are directly aware of material things?’ (3)
The argument is based on the possibility of illusion and hallucination. The meanings of the terms ‘hallucination’ and ‘illusion’, and the difference between the two.
Despite Ayer’s use of the phrase ‘argument from illusion’, the argument he presents would be better described as the ‘argument from hallucination.’ It may be formalized as follows:
| 1. | All experience have an object. |
| 2. | In the case of hallucination, the object of experience cannot be a material object. |
| 3. | The objects of experience are the same in the case of hallucinatory and veridical experience. |
| C. | Therefore, material objects are never the objects of experience. |
But we can also give an argument for the same conclusion based on the possibility of illusion rather than hallucination.
Moore’s version of the argument from illusion (in ‘A defence of common sense’). The example of the envelope. Moore’s argument gets going in more standard cases of perception than Ayer’s, but also needs a stronger first premise.
| 1*. | Whenever it seems to you that you are perceiving something F, there is something F that you are perceiving. |
| 2*. | In the case of an illusory experience of some object as F, there is no material object which is F which could be the object of experience. |
| 3*. | The objects of experience are the same in the case of illusory and veridical experience. |
| C. | Therefore, material objects are never the objects of experience. |
The conclusion of Ayer’s argument follows from the premises, and premise 2 is not open to dispute. So the key premises for evaluating the argument are P1 and P3. Ayer spends most of his time defending P3, so we will focus on that one. (We will discuss P1 when we turn to objections to the argument.) Ayer gives three arguments in favor of P3.
Ayer suggests that if the objects of veridical and illusory perceptual experiences were different kinds of things, we should expect this to issue in some qualitative difference in the perceptual experience. But it does not:
“When I look at a straight stick, which is refracted in water and so appears crooked, my experience is qualitatively the same as if I were looking at a stick that really was crooked. When, as the result of putting on green spectacles, the white walls of my room appear to me to be green, my experience is qualitatively the same as if I were perceiving walls that really were green.” (6)
The fact that veridical and illusory perceptions “may form a continuous series, both with respect to their qualities and with respect to the conditions under which they are obtained.” (8)
The example of approaching an object from a distance; the problem with this example; the meaning of ‘veridical perception’ when the property in question is the size of an object.
A better example: alteration in the color of a wall under changing lighting conditions.
Note: all of these cases involve illusion, rather than hallucination. An example involving hallucination; ‘veridical hallucinations.’ Lying in a hospital bed looking at lights above you.
The objects of perception are dependent upon the state of one’s nervous system, whereas material objects typically are not:
“...the relation between my perception and these accompanying conditions [such as lighting, the state of my brain, and other facts which ‘enable’ the perceptual experience in question] is such that, while they are not causally dependent on it, it is causally dependent upon them. ...This point being established, the argument proceeds as follows. It is held to be characteristic of material things that their existence and their essential properties are independent of any particular observer. For they are supposed to continue the same, whether they are observed by one person or another, or not observed at all. But this, it is argued, has been shown not to be true of the objects we immediately experience. And so the conclusion is reached that what we immediately experience is in no case a material thing.” (10-11)
An apparent flaw in the argument: from the fact that our perceptions are causally dependent on our own physiological states it does not follow that the objects of our perceptions are so dependent.
An argument from the principle that an object must exist at a time t to be perceived at t.
The implausibility of the analogous principle in the case of belief. Why the principle can nonetheless seem plausible in the case of perception; the case of afterimages.
It may be plausible to reject this principle for cases of hallucination.
A problem with giving up this principle: hallucinations as explaining facts about our mental lives.
The idea that cases of veridical perception, illusion, and hallucination do not form a unified class to be given a single treatment.
Problems with this response to the argument: (i) the possibility of hallucinations indistinguishable from any given veridical experience; (ii) sameness of brain activity in cases of veridical perception and cases of hallucination.
Weirdness-based worry: the very idea of private mental objects which are constantly coming into and going out of existence.
Materialism-based worry: the difficulty of accounting for sense data within the confines of a materialist theory of the mind.
Sense data as objects who have only the properties they are perceived to have, or seem to have. Why it is natural to hold that sense data have this property. Problems with the idea that sense data have this property.
One worry about the philosophy of perception encouraged by Ayer’s argument focuses not on intrinsic difficulties in understanding sense data, but rather on the role that perception plays in justifying beliefs.
It is obvious that, in many cases, perceptual experiences can justify beliefs about everyday objects like chairs, desks, other people, etc. But, on a sense datum theory of perception, the question arises: given that we are only acquainted with sense data in perception, how could perception justify beliefs about external objects?
The only obvious answer to this question is: by inference from our experience of sense data. But this gives rise to the question: how is this inference from experience of sense data to beliefs about external objects itself justified? (Not, obviously, on the basis of some observed correlation between sense data and those external objects, since our experience is only of sense data.)
A radical solution to this problem, and the phenomenalist idea that material objects are ‘logical constructions out of sense data’ or, in Mill’s phrase, ‘permanent possibilities of sensation.’ Link to Russell’s logical atomism.
Empiricist views of concepts + the sense datum theory of perception. The importance of Russell’s theory of descriptions here.