=Habit= is, in general, the outcome of practice; if practice shows us a nervous set or disposition in the making, habit is the set taken, the disposition established; the plastic organ has hardened in some special way. Like practice, habit in its early stages requires _attention_; but it is to be noticed that a habit may be formed, not only by the repetition that practice brings, but also by any single stimulus that violently impresses the nervous system; the plastic mechanism may be thrown, by a sudden wrench, into a new and permanent arrangement; just as we may give a permanent bend to a fencing foil by a single violent lunge. We have already seen, in our discussion of the development of attention (p. 99), that habits already formed are the basis of new acquisition; and we may remark in passing that the moral and practical importance of habit has often been written upon and can hardly be overestimated. In all experimental work of a serial kind, habit shows itself as a _tendency to experience and report the same things_. Suppose, for example, that we wish to ascertain the least perceptible difference of tonal pitch. We begin with two identical tones, and gradually separate them in the successive experiments of the series. The observer begins with the experience and report of ‘same’ or ‘alike.’ If, now, the differences between the tones are made very small, so that the series of observations is long drawn out, the observer may get into the habit of hearing and reporting ‘same’; although the tonal difference is definitely perceptible, it nevertheless passes without notice. The focal processes are here, as they are in the case of practice, few in number; but they run their course at a low level of attention; they are intrinsically obscure, and the report of them simply follows the line of least resistance. The observer is correspondingly liable to distraction from the outside; the inhibitory protection is weak. Habituation is consequently to be avoided, as practice is to be desired. =Fatigue= appears to be due to a sort of blood-poisoning; waste-products thrown off by the other tissues are poured into the blood-stream and there accumulate. It shows itself first of all by way of _muscular sensation_ (p. 46), and soon becomes a sense-feeling; whereupon the biological theory of feeling lays hold of it (p. 84), and bids us stop work because we are suffering harm. _The feeling of fatigue, however, gives no sure evidence that the capacity of the nervous system is reduced_; the biological theory signally breaks down; not only can we work effectively, but we often do our best work, after we have begun to feel tired. We should take our cue to rest not from the feeling of fatigue, but rather from the impairment of our work, in quantity and quality, on the one hand, and from derangement of the great bodily functions, such as digestion and sleep, on the other.