It is not possible to make a logically sharp distinction in this matter; in certain cases we have to be satisfied with something less exact. For example, we might say that those developments which are merely due to normal growth are to count as unlearned, while those which depend upon special circumstances in the individual biography are to count as learned. But take, say, muscular development: this will not take place normally unless the muscles are used, and if they are used they are bound to learn some of the skill which is appropriate to them. And some things which must certainly count as learned, such as focussing with the eyes, depend upon circumstances which are normal and must be present in the case of every child who is not blind. The whole distinction, therefore, is one of degree rather than of kind; nevertheless it is valuable. The value of the distinction between learned and unlearned reactions is connected with the laws of learning, to which we shall come in the next chapter. Experience modifies behaviour according to certain laws, and we may say that a learned reaction is one in the formation of which these laws have played a part. For example: children are frightened of loud noises from birth, but are not at first frightened of dogs; after they have heard a dog barking loudly, they may become frightened of dogs, which is a learned reaction. If we knew enough about the brain, we could make the distinction precise, by saying that learned reactions are those depending upon modifications of the brain other than mere growth. But as it is, we have to judge by observations of bodily behaviour, and the accompanying modifications in the brain are assumed on a basis of theory rather than actually observed. The essential points, for our purposes, are comparatively simple. Man or any other animal, at birth, is such as to respond to certain stimuli in certain specific ways, _i.e._ by certain kinds of bodily movements; as he grows, these ways of responding change, partly as the mere result of developing structure, partly in consequence of events in his biography. The latter influence proceeds according to certain laws, which we shall consider, since they have much to do with the genesis of “knowledge”.