§ 8. Ideas accompanying Perceptual Process.— So far we have treated of perception and perceptual process in its pure form. We have distinguished it sharply from ideational process. But in the actual mental life of man the two run into one another, so that we do not usually find pure perceptual processes, but rather what we may call perceptual processes not absolutely, but only a potiori. The same is true to some extent of the higher animals also. Free ideas may accompany a process without interfering with its essentially perceptual nature. The free ideas may fulfil a function essentially analogous to that fulfilled by perception, and not any function which by its very nature requires the presence of ideas. This happens when the only office discharged by mental imagery is to prompt or guide the execution of an action, and not to lay out the plan of an action beforehand in the form of a train of thought. Mr. Batchelder's squirrels gnawed at the nuts and by reaching their contents satisfied their congenital craving without any mental image of the kernel inside. Suppose that on a future occasion they start with this mental image, the character of the process is not essentially altered. The image of the kernel inside now only contributes to prompt and guide the action, just as the mere perception of the nut prompted and guided it before. Free images may be especially useful and even necessary in this way, when the activity is comparatively complicated, and undetermined by definite congenital impulses. Take for instance the case of a monkey imitating a train of actions which it has seen performed by a man, — those concerned in shaving, for instance. Possibly percepts would alone suffice in such a case. The sight of the razor might prompt the act of sharpening it, and the act of sharpening it might next prompt the lathering, and so on. But certainly it is easier to understand the action if we suppose that in different phases of its progress some mental image of the behaviour of the man arises in the mind of the monkey, and helps to guide him.

It would seem that in animals ideas, so far as they exist* at all, are isolated and, so to speak, sporadic. They do not as a rule give rise to further ideas following each other in a train. Their function is rather to guide the development of a motor impulse as percepts guide it. As Mr. Thorndike says, the impulse and not the idea is the essential thing.

* There is room for difference of opinion on this point. Personally, I do not think that there is much evidence for the presence of ideal images in the animal mind, except in the case of the more intelligent monkeys and perhaps of elephants.

In our own mental life, free ideas are almost constantly present, so that purely perceptual activity is comparatively exceptional. But it certainly takes place. If I have once been bitten by a dog, and meet the same dog on another occasion, I do not need to summon up in my mind a mental image of being bitten again in order to take practical measures of an intelligent kind.

The vast interval which separates human achievements, so far as they depend on human intelligence, from animal achievements, so far as they depend on animal intelligence, is connected with the distinction between perceptual and ideational process. Animal activities are either purely perceptual, or, in so far as they involve ideas, these ideas only serve to prompt and guide an action in its actual execution.* On the other hand, man constructs "in his head," by means of trains of ideas, schemes of action before he begins to carry them out. He is thus capable of overcoming difficulties in advance. He can cross a bridge before he comes to it.

* There may be exceptions to this rule, hut the general statement is broadly true.