_Attention_ is again necessary. Intention, on the other hand, seems not to be necessary; there need be no special purpose behind the experiences, as the intention to learn is behind the experiments with meaningless syllables; attention is enough. The idea of a surgical operation, for instance, may be permanently connected with the idea of the surgeon who performed it, although the intervention of that particular surgeon was quite casual and unexpected. The reason is that _attention brings a situation, its own situation, with it_; the determinants of primary attention are, as we put it on p. 97, the ‘great biological stimuli,’ things that an organism _must_ take notice of, if it is to persist as a living organism at all; and the determinants of derived primary attention are also what we may call ‘situational’ affairs, things that appeal in certain circumstances to certain sides of our nature, things that interest or ‘impress’ us. So attention, too, implies a set or disposition of the nervous system; common sense is so far in the right—though its words are misleading—when it talks of a ‘concentration of the mind,’ of ‘pulling oneself together,’ and the like; and _this general set is sufficient_, without the presence of a distinct purpose. Our law will read, then, somewhat to this effect: _If a number of vivid perceptions or ideas, whose situational context is the same, occur together under favourable conditions, then the later appearance in the same situational context of any one will tend to be accompanied, according to circumstances, by the reappearance (as ideas) of the others_.