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A.J. Ayer's Presentation of the Problem of Sense Data

Because of the possibility of illusion, it will not necessarily be true that whenever it seems to me that I am perceiving something, I am really perceiving it. On the other hand, the converse is intended to hold. From the statement that I see the cigarette case it is supposed to follow that it seems to me that I see it. Or, if this cannot be maintained, it is at least supposed to follow that it seems to me that I see something or other. It is to be a necessary fact that whenever anything is perceived something must, in this sense, seem to be perceived. But whether this entailment really holds is a question which we shall have presently to examine.

The next step, continuing with our example, is to convert the sentence 'it now seems to me that I see a cigarette case' into 'I am now seeing a seeming-cigarette case'. And this seeming-cigarette case, which lives only in my present experience, is an example of sense-datum. Applying this procedure to all cases of perception, whether veridical or delusive, one obtains the result that whenever anyone perceives, or thinks that he perceives, a physical object, he must at least be, in the appropriate sense, perceiving a seeming-object. These seeming-objects are sense-data; and the conclusion may be more simply expressed by saying that it is always sense-data that are directly perceived.

If this conclusion is allowed to be legitimate, it still does not follow that näive realism is false. The näive realist can, indeed, be refuted if he is made, as by Professor Price,¹ to adopt the view that visual and tactual sense-data are parts of the surfaces of the physical objects. All that is then needed is to point out that the properties which are, by definition, ascribed to the surfaces of physical objects are inconsistent with those that are ascribed to sense-data. For instance, the surface of a physical object can exist without being perceived, but this cannot be said of sense-datum. Price forces this unteneable position upon the näive realist because he attributes to him the view that physical objects are directly perceived; and so, being himself persuaded that sense-data are directly perceived, he concludes that the näive realist must be maintaining that sense-data and physical objects are somehow identical. But, the näive realist, if he is circumspect, will not distinguish in this way between direct and indirect perception. His thesis must be that our ordinary way of speaking, in which this distinction is not made, is perfectly adequate for describing all of the facts; and we have indeed seen that this standpoint can be maintained. All the same, if the procedure which leads to the introduction of sense-data is legitimate, the näive realist by refusing to follow it denies us an insight into the analysis of perceptual statements. His method of describing the facts, though adequate in one sense, is not in another: for there are important distinctions which it fails to bring to light. It has, indeed, the advantage that it shields us from a difficult problem; there is no question for the näive realist of anything's ever seeming to come between us and the physical world. But philosophical problems are not settled simply by taking care that they should not arise. If the introduction of sense-data is permissible, then there exists a problem about the way in which they are related to physical objects. If this question can be raised, it is philosophically entitled to an answer.

But can it be raised? The steps which are supposed to lead us to talk about sense-data are each of them open to challenge. Consider first the claim that in making such a statement as I see my cigarette case, I assert more than is strictly warranted by the content of my present experience. This may well provoke the objection that it is not at all obvious that what we are to understand by such an expression as 'the content of my present experience'. If I am asked what experiences I am having at this moment, and if I interpret this somewhat unusual question as requiring me to say, among other things, what it is that I see, I shall answer quite correctly if I say that, among other things, I see a cigarette case. But, if this answer is correct, then, in saying that I see a cigarette case, I am not doing more than describe my present experience. Yet it does seem that there is a sense in which I could be having just this experience, even though I was not seeing any physical object at all. But if this is to be said, we must give 'the content of experience' a narrower interpretation. We must take it to refer, in this instance, only to what is 'visually given' to me, irrepective of its connection with anything else. The question is whether such an interpretation is intelligible.

1. H. H. Price, Perception (London, 1932) ch. 2

Taken from A. J. Ayer, The Problem of Knowledge [Penguin Books, 1990], pp. 96 - 98.

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