What is sense data in philosophy?

Bertrand Russell, answered from the texts and cited to the page.

Bertrand Russell answers

Sense-data are the immediate objects of sensation — the brown colour, the oblong shape, the smoothness that one encounters when one looks at or touches a table.¹ They are distinct from the physical object itself, which, if it exists at all, is not immediately known to us but must be inferred from what is immediately given.² The colour is that of which we are directly aware; the awareness itself is the sensation.

Sense-data and sensation are not the same thing, and keeping them apart matters. What can we learn by observation and experiment? Nothing, so far as physics is concerned, except these immediate data: patches of colour, sounds, tastes, smells, with their spatiotemporal relations.³ The molecules and electrons of physics have no colour, make no noise, and are in principle never directly encountered.

They are inferred from sense-data, not observed alongside them, which creates a genuine difficulty for any claim that physics is empirically verified — since only one term of the supposed correlation, the sensible term, is ever actually found.⁴ The epistemological point follows directly. Sense-data are all that we directly and primitively know of the external world; the fact that they are data is all-important for epistemology.⁵

But — and this is worth stating plainly — the fact that they are all we directly know gives no presumption that they are all there is. My knowledge of the table as a physical object is knowledge by description: the table is "the physical object which causes such-and-such sense-data," and the object itself is never directly before the mind at all.⁶

All our knowledge, both of things and of truths, rests upon acquaintance as its foundation, and sense-data supply the most obvious example of knowledge by acquaintance; but if they were the sole example, our knowledge would be far more restricted than it is.⁷ Berkeley saw part of this clearly enough — he showed that the existence of matter can be denied without absurdity, and that whatever exists independently of us cannot be the immediate objects of sensation.⁸

Where I part from him is in refusing to conclude that matter therefore does not exist. The inference from "not immediately known" to "not there" is one I have never been persuaded to make.

Sources

1. 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.

   The Problems of Philosophy

2. The real table, if there is one, is not immediately known to us at all, but must be an inference from what is immediately known.

   The Problems of Philosophy

3. Nothing, so far as physics is concerned, except immediate data of sense: certain patches of colour, sounds, tastes, smells, etc., with certain spatiotemporal relations.

   The Collected Papers Vol 8, The Philosophy of Logical Atomism 1914-19, pp. 51–52

4. A correlation can only be ascertained empirically by the correlated objects being constantly found together. But in our case, only one term of the correlation, namely the sensible term, is ever found: the other term seems essentially incapable of being found.

   The Collected Papers Vol 8, The Philosophy of Logical Atomism 1914-19, pp. 51–52

5. Sense-data at the times when they are data are all that we directly and primitively know of the external world; hence in epistemology the fact that they are data is all-important.

   The Collected Papers Vol 8, The Philosophy of Logical Atomism 1914-19, p. 52

6. My knowledge of the table as a physical object, on the contrary, is not direct knowledge... The table is 'the physical object which causes such-and-such sense-data'.

   The Problems of Philosophy

7. All our knowledge, both knowledge of things and knowledge of truths, rests upon acquaintance as its foundation... sense-data, as we have already seen, are among the things with which we are acquainted; in fact, they supply the most obvious and striking example of knowledge by acquaintance.

   The Problems of Philosophy

8. Berkeley retains the merit of having shown that the existence of matter is capable of being denied without absurdity, and that if there are any things that exist independently of us they cannot be the immediate objects of our sensations.

   The Problems of Philosophy