H. P. Grice cites H. H. Price, "Perception," 1932.

If the argument from hallucination is sound, then no aspect of sense perception can be as the Naïve Realist claims. But that leaves open which other account of sense perception one should adopt. That is, the argument from hallucination alone provides no argument for which theory you should accept but only for what theory you should reject. 1. What are sense-data? Naïve Realism claims of veridical sense perception that one’s sensory experience is: i.) a relation to some entity; ii.) the entity in question is mind-independent, e.g. a table or chair Sense-datum theories of perception accept (i) but reject (ii).

When I see a tomato there is much that I can doubt.

I can doubt whether it is a tomato that I am seeing, and not a cleverly painted piece of wax. I can doubt whether there is a material thing there at all…

One thing however I cannot doubt: that there exists a red patch of a round and somewhat bulgy shape, standing out from a background of other colour-patches, and having a certain visual depth, and that this whole field of colour is presented to my consciousness…that something is red and round then and there I cannot doubt…that it now exists, and that I am conscious of it—by me at least who am conscious of it this cannot possibly be doubted…

This peculiar and ultimate manner of being present to consciousness is called being given, and that which is thus present is called a datum. (H. H. Price, Perception, 1932, p.3.)

According to sense-datum theorists, sense-data are whatever is given to the mind when one senses. It is not definitional for them that a sense-datum is non-physical or non-material. Rather, philosophical argument is supposed to show that material objects cannot be given to the senses. Note two assumptions in Price’s approach: a.) that there is always something which is given (i.e. he accepts (i) above or Robinson’s Phenomenal Principle); b.) that there is something interesting in common among the entities which are given – they share a presentative function. (Compare, should we talk of stomacha-data to mean, those things given to digestion?) 2. What do sense-data do? They are posited to explain some or all aspects of how things appear to a subject when that subject enjoys a sense experience. Things look (or more generally, appear) a certain way to someone, according to the sense-datum theorist, when a sense-datum with appropriate properties is presented to them. Can they explain all aspects of how things look? When it looks to you as if there is a rabbit in front of you, does the sense-datum theorist posit a sense-datum rabbit? If not, which aspects of sense-experience are explained by sense-data and which by something else? (Cf. also the Problem of the Speckled Hen discussed below.) 3. Two further issues (These divide different kinds of sense-datum theory): (A) Does one see a visual sense-datum in the same sense in which one sees a table or chair? Moore (‘Visual Sense-Data’) says ‘Yes’; so does Jackson, hence Jackson’s employment of the distinction between immediate and mediate objects of sight. According to Jackson, visual sensedata are always the immediate objects of sight in virtue of which one sees anything else. 1 BUT a sense-datum theorist can claim that sense-data are responsible for fixing the character of how things appear to us, and are the objects of awareness without supposing that we see them. E.g. they may claim that seeing necessarily involves the activity of the eyes, or causal interaction with the environment, or interaction with light. (B) What is the status of non-physical sense-data? Are they mind-dependent or not? The traditional sense-datum theorists – Moore, Price (and Russell and Broad) all insisted that sense-data are mind-independent as well as non-physical. (Why did they claim this? One answer is that Moore took sensing to be a form of knowledge – Russell too talks of knowledge by acquaintance – and claimed that knowledge is possible only of entities independent of the knowing of them.) As we have seen, this view of sense-data a.) undermines the argument from hallucination against Naïve Realism; b.) conflicts with the assumption that sense experience is subject only to physical and psychological causes. In later writers, e.g. Jackson, it is not assumed that sense-data are mindindependent. 4. Objections to Sense-Data Critics of sense-data have generally objected to them on three kinds of grounds: a.) Ontological: To suppose the existence of such entities conflicts with our broader metaphysical commitments – e.g. physicalism; or is to commit to the existence of objects with inconsistent properties; or is to commit to redundant objects (the issue of adverbialism); b.) Epistemological; To posit the existence of sense-data is to hypothesise a kind of veil of perception which prevents us from having knowledge of the physical world; or being able to think about or single objects out; c.) Phenomenological; To posit the existence of sense-data is inconsistent with how we think about what our sense experience is like: such experience seems to involve an awareness of physical objects and their qualities, and does not involve the seeming presence of non-physical objects or qualities that they possess. 5. The Ontological Objections to Sense-Data A. Physicalism: Is physicalism inconsistent with the existence of sense-data? How should we define physicalism? If we say that all entities that exist should either be identical with physical entities or made up out of physical entities, then the existence of sense-data is inconsistent with physicalism. What are physical entities? One suggestion: the concrete entities we commonly single out in the world around us, tables or chairs. Another suggestion: the entities posited by our best physics – e.g. field phenomena. Where do rainbows, shadows, holes, and sounds fit into this story? If physicalism is the doctrine that all facts are physical facts – e.g. that any world which duplicates this world in terms of the facts about the physical world, thereby duplicates all facts about the world – then it is less clear that positing sense-data is inconsistent with the truth of physicalism. B. Are sense-data just too weird for us to accept that they exist? The Problem of the Speckled Hen Suppose your eye sight is not that good: you may see a hen to be speckled, and yet there be no determinate number of speckles which you then see the hen to have. (Compare: you look at a crowd in Haas Pavilion – you can see that there is a huge crowd of people, but do you see the exact number?) How many speckles are there in the sense-datum of which you are aware, according to the sensedatum theorist