There is one more point to which attention may be drawn. If there be one thing, and there certainly are not many, on which all writers on the emotions are agreed, it is as to their vagueness. They do not readily submit to definition, and cannot be described in a sentence. This is not due to any indefiniteness of biological end, nor to much indefiniteness in the mode of “expression;” it is due, rather, to an inherent dimness and haziness of psychological outline. We seem unable to focus them and get a clear-cut result. This is, no doubt, in part due to the complexity of emotional states. But, may it not be largely due to the fact that there is no necessity for definiteness? They fulfil their purpose just as well if they are vague. It is quite necessary for the dog to have a clear-cut impression of his antagonist; and, on the cognitive side of consciousness, meaning must be in some degree definite to be of real value. But, so long as the emotion raises the temperature, so to speak, to the boiling-point of vigorous action, it matters little what the psychological source of heat may be. If this be so, we should expect an emotional vagueness, since natural selection puts no premium upon emotional definiteness. And from this it follows, as a corollary, that, whereas we may infer that an animal’s perceptual products are probably closely similar to our own, since sight, touch, hearing, smell, and taste are of value in so far as they convey definite meaning, in interpreting their feelings and emotions we have less secure grounds of inference, since all that is requisite is that there should be a sufficiently high emotional temperature to afford the conditions for definite and vigorous action. In conclusion, then, we may say that the primary purpose of the evolution of feeling and emotion is to promote beneficial behaviour, and that the observed consonance of the psychological end of attaining satisfaction, and the biological end of securing survival, seems to be due to natural selection--is, indeed, scarcely explicable on any other naturalistic hypothesis. A word of warning may be added. We have repeatedly spoken of biological and psychological ends. By this we mean what seems to the observer, as an interpreter of natural processes, the purpose and object of their existence. But the word “end” is often used in such a way as to imply foresight and contrivance on the part of a rational being. We have not used it in this sense. Whether the whole of nature, including animal behaviour, is driven onwards to definite ends by an underlying Cause, is a metaphysical question. It is not one on which science has any right to express an opinion one way or the other. Science deals with the phenomena; the causes of their being lie outside her province. CHAPTER VII _THE EVOLUTION OF ANIMAL BEHAVIOUR_ I.--THE PHYSIOLOGICAL ASPECT