Sense-Data

For Locke and Hume, and British empiricists generally, the way to understand any psychological concept is either to find it among the immediate data of introspection or to show how it is to be analyzed into such data. This approach ultimately stems from the Cartesian insistence that one knows one’s own states of consciousness better than anything else, in particular, better than physical objects and events, since it is possible to doubt the existence of all the latter but not all of the former.

William Alston, Pleasure, in Paul Edwards (ed.) The Encyclopedia of Philosophy, New York, 1967, pp. 341–347, p. 622

I may be wrong in believing that matter exists independently of me. But the suggestion that I am wrong in believing I have a sensation is absurd. The belief is not sufficiently separable from the sensation for the possibility of error. I may, of course, be wrong in believing that I had a sensation in the past, for memory may deceive me. And I may be wrong in the general terms which I apply to a sensation, when I attempt to classify it, and to describe it to others. But my knowledge that I am having the sensation which I am having is one of those ultimate certainties which it is impossible either to prove or to deny.

J. Ellis McTaggart, Human Immortality and Pre-Existence, London, 1916, pp. 26-27