We must, therefore, base our method of procedure on some definite principle. The canon of interpretation which I have elsewhere suggested[158] is, that we should not interpret animal behaviour as the outcome of higher mental processes, if it can be fairly explained as due to the operation of those which stand lower in the psychological scale of development. To this it may be added--lest the range of the principle be misunderstood--that the canon by no means excludes the interpretation of a particular act as the outcome of the higher mental processes, if we already have independent evidence of their occurrence in the agent. Now, the conclusion to which we are led by direct experiment and a critical study of the actions of animals whose life-history is known to us is, that most of their behaviour--perhaps all--is due to what Dr. Stout terms the perceptual, as opposed to the ideational, exercise of cognition. Their behaviour can be explained without having recourse to the hypothesis that they reflect, and attain to ideal schemes as the result of abstraction and generalization consciously directed to this end. Rather than repeat what I have already said, I will quote Dr. Stout’s summary of the position to which he, too, has been led. “The vast interval,” he says,[159] “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 serve only 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.”