 By sense-data I understand a class of entities of which we are very often directly conscious, and with many of which we are extremely familiar. They include the colours, of all sorts of different shades, which I actually see when I look about me; the sounds which I actually hear; the peculiar sort of entity of which I am directly conscious when I feel the pain of a toothache, and which I call “the pain”; and many others which I need not enumerate. But I wish also to include among them those entities called “images”, of which I am directly conscious when I dream and often also when awake; which resemble the former in respect of the fact that they are colours, sounds, etc.; but which seem, as a rule, like rather faint copies of the colours, sounds, etc., actually seen or heard, and which, whether fainter or not, differ from them in respect of the fact that we should not say we actually saw or heard them, and the fact that they are not, in the strictest sense of the words, “given by the senses”. All these entities I propose to call sense-data. And in their case there is, of course, no question whether there are such entities. The entities meant certainly are, whether or not they be rightly described as “sensations”, “sense-presentations”, “sense-data”, etc. Here the only question can be, whether they are “mental”. (1909–10: 57)