§ 2. Unity and Continuity of Perceptual Process. Many perceptions are very brief and evanescent. They satisfy a slight and momentary interest, after which the mind passes to other occupations. Other perceptions do not occur in this isolated manner: but enter into more prolonged trains of mental activity as constituent moments or phases. These more prolonged trains may be mainly trains of ideas: but they may also be mainly composed of a sequence of perceptions. A man climbing a precipitous cliff may have his attention fully occupied in gaining and retaining foothold and handhold. His activities mainly consist in muscular movement guided by senseperception. Such an act as threading a needle does not necessarily involve ideal images; attention is fully occupied in the guidance of the hand, and the delicate coordination of its movements by the aid of the eye. The same holds broadly true of such performances as walking on a tightrope, keeping one's balance on a bicycle so far as it may require attention, and, in general, of games of bodily skill. In these instances, perceptions are not isolated facts; they form series having a certain unity and continuity similar to that of trains of ideas or trains of thought. Any such series constitutes a single complex perceptual process. It differs from a train of ideas inasmuch as the sequence of its parts does not depend on direct mental reproduction. The sequence of its parts depends upon the sequence of external impressions; but as the sequence of external impressions depends to a very large extent upon the bodily movements of the percipient subject, it is to a very large extent under subjective control.

It is in these complex forms that the distinctive characteristics of perceptual process can best be studied. In many ways, the best field for their study is animal life. They are found in definite forms in the instinctive activities of animals, viz. in those activities for which the animals are predisposed and preadapted by the inherited constitution of their nervous systems. They frequently arise at a period in the life of the animal at which it has had no opportunity for the acquisition of corresponding ideas; so that there can be no doubt concerning the predominantly or purely perceptual character of the process. For instance, the train of actions involved in hunting a living prey is shown in the play of the kitten before it has actually hunted, and often without its having had opportunity for learning them by imitation.* The kitten will first assume the attitude of watching or lying in wait; it will then steal up to the ball of thread or other object which forms its plaything, in a noiseless snakelike manner; in the next place it gathers itself for a spring, and pounces on the quasiprey, seizes it with teeth and claws, and worries it; finally it lets the object go again, and recommences the process. The several acts of lying in wait, stealthy approach, crouching for a spring, pouncing on the prey, are phases in the development of the same activity. The same is true of the hunting of an actual prey.

* We have no data which would enable us to characterise precisely the cognitive attitude of an animal towards the object which for the first time calls into play a train of instinctive movements having the internal continuity which marks perceptual process. We cannot say the animal recognises the object, for recognition presupposes previous experience. But the mental attitude is probably more analogous to what we call recognition than to anything else. There is no preparation by previous experience, but there is preparation by congenital endowment, which seems to fulfil an analogous function.

All such processes are guided by external impressions; but each impression, as it occurs, only supplies the occasion for the further development of an activity which is already in existence. If the series were one of purely reflex actions, each separate stimulus would independently produce an isolated reaction, so that the process would have no internal continuity. But it is just the internal continuity which is distinctive of perceptual activity. The successive phases of perceptual process are directed to one end, and this end is not merely nature's end; it is what the animal itself is in some sense striving after, or, if we may use such an expression, what it is "driving at." This does not necessarily mean that the animal presents to itself a mental image of the result to be effected. It means that the felt tendency, conation, or endeavour, with which the train of movements starts, is not satisfied until the end is reached. Like the baby in the advertisement of Pears' soap, the animal "won't be happy till he gets it." With the final attainment of the result, the conation ceases, because it has worked itself out. Interruption, delay, or failure at any stage of the process is a thwarting of the one continuous conation; it is felt as displeasure and aversion, and is accompanied by a tendency to vary the mode of procedure. The successful progress of the action is in every stage felt as a pleasure and as a tendency to continue in the same course. Stated from the physiological point of view, what happens is as follows. Given a neural arrangement for the coordination of successive acts, the equilibrium of this arrangement is disturbed by stimulation either from without the organism or from within. It can only recover its own balance and so become quiescent by a series of successive processes leading to a certain result. By these its initial excitement is allayed. The psychical activity whereby a conation develops itself, and so brings about its own end or termination, is the counterpart of the activity whereby the neural system regains equilibrium.

This unity and continuity of perceptual process is its most general feature. The following special characteristics may be regarded as bringing out in detail from various points of view what is involved in this general account.