In the present state of philosophy in the English-speaking world, to choose to talk about sense data may seem perverse. What could be more boring for one's audience than to attempt variations on so threadbare a theme? And worse, what could be more unfashionable in the aftermath of Wittgenstein and Austin? My reasons for selecting this unpromising topic are twofold. First, the general theme of this series of lectures is empiricism. And whatever meanings we put upon that ambiguous word, it is clear that as a matter of history the problems of perception have been important problems for nearly all those philosophers who would consider themselves to be empiricists. And however unsatisfactory sense datum theories of perception may now be held to be, such theories have been central to the empiricist tradition. Secondly, it is important not to be too much impressed by the fact that a particular philosophical opinion is fashionable or unfashionable. The former certainly does not guarantee its truth nor the latter its falsity. It has often been remarked that philosophical opinions are very rarely refuted. Instead they fall out of vogue only to return some years later in another guise. It is perhaps time to take another look at the notion of sense data. The most ingenious and persistent attacks on analyses of perception in terms of sense data have been at best indecisive, as Professor Ayer showed in his reply to Austin's Sense and Sensibilia.