(3) _Derived primary attention._—One of the strongest proofs that there is no real difference between primary and secondary attention is that, in course of time, these difficulties vanish. Habit, as we say, becomes second nature; the thoughts that at first moved haltingly and with all sorts of interruption gradually become absorbing; work that was once done with pains and labour grows fascinating, and makes an unquestioned demand upon us. So the period of struggle ends, and _we slip back again into primary attention; only this derived form is controlled, not by the great biological stimuli, but by impressions that fit in with our acquired interests_. The collector, the inventor, the expert are roused to keen attention by stimuli which the rest of the world pass without special notice. Most of the striking coincidences of life are accounted for by this law; you are thinking about certain things, and something happens that, because you are thus thinking and because it is akin to the subject of your thought, captures your attention. ‘What an amazing coincidence!’ you cry; but if you had been occupied with some other topic, there would have been no coincidence. The man in Mr. Kipling’s story who wondered, years after the event, ‘how in the world he could have written such good stuff as that’, had written under this same law of attention; for when you are thoroughly absorbed in a subject, relevant facts and ideas crowd upon you; the mind stands open to them, while it is fast locked against the irrelevant; and you surpass yourself. There is, to be sure, another side to the picture; the enthusiastic adoption of a belief or theory throws into brilliant relief all the facts that tell in its favour, but blinds you to the considerations that make against it.