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.

John McTaggart, Human Immortality and Pre-existence, London, 1916, pp. 26-27