began. Nay, more, they have all displayed the national characteristics of the observer. Animals studied by Americans rush about frantically, with an incredible display of hustle and pep, and at last achieve the desired result by chance. Animals observed by Germans sit still and think, and at last evolve the solution out of their inner consciou ness. To the plain man, such as the present writer, this situation is discouraging. I observe, however, that the type of problem which a man naturally sets to an animal depends upon his own philosophy, and that this probably accounts for the differences in the results. The animal responds to one type of problem in one way and to another in another; therefore the results obtained by different investigators, though different, are not incompatible. But it remains necessary to remember that no one investigator is to be trusted to give a survey of the whole field.
The matters with which we shall be concerned in this chapter belong to behaviourist psychology, and in part to pure physiology. Nevertheless, they seem to me vital to a proper understanding of philosophy, since they are necessary for an objective study of knowledge and inference. I mean by an "objective" study one in which the observer and the observed need not be the same person; when they must be identical, I call the study "subjective". For the present we are concerned with what is required for understanding "knowledge" as an objective phenomenon. We shall take up the question of the subjective study of knowledge at a later stage.
The scientific study of learning in animals is a very recent growth; it may almost be regarded as beginning with Thorndike's Animal Intelligence, which was published in 1911. Thorndike invented the method which has been adopted by practically all subsequent American investigators. In this method an animal is separated from food, which he can see or smell, by an obstacle which he may overcome by chance. A cat, say, is put in a cage having a door with a handle which he may by chance push open with his nose. At first the cat makes entirely random movements, until he
