The concept of purpose is one which appears to be of fundamental importance for the theory of motivation. This concept has, however, been the object of radical disagreement among psychologists. Some of the latter have regarded it as furnishing the clue to all psychological thinking, whereas others have rejected it entirely as a basic explanatory principle. Mechanists have naturally objected to accounts in terms of purpose, whereas more sentimental thinkers have found this notion the easiest one to use in their attempts to solve the problems of human motivation. For a long time, philosophers have contrasted explanations of the purposive type with those in terms of mechanical causation; they have implied that the distinction between so-called final and efficient causes is fundamental, and that the two notions are incapable of being reduced to a common denominator.
It is our view that this conflict between the purposive and the mechanistic theories is attributable to a confusion of ideas and to superficiality of analysis. Purpose, as it is understood by the man-in-the-street, is undeniably a factor in life, and no amount of introspection or physiological research can rule it out. A psychology which eliminates purpose cannot possibly correspond with the facts. However, the man-in-the-street cannot tell you what purpose actually is, although it is quite likely that he will give an account of it which closely resembles that which is offered by the purposive psychologist or philosopher. The commonest confusion is of course that between purpose and function. Probably no enlightened modern thinker intends to identify these two ideas, but their complete separation appears to present emotional difficulties. The mere fact that a structure is subservient or instrumental to the maintenance of life seems to endow it with purpose.
The exact import of Darwin's teachings has probably been missed even by many of his most devout adherents. Possibly he did not perceive it in its broadest aspects, himself, since he was concerned to show that the purposive account of organic structure and function could be superseded, instead of dealing directly with the facts of evolution, without any reference to purpose. The essence of Darwin's formula lies in a proposition which seems almost tautological. It says that only those structures can continue to exist which are so constituted as to make them capable of existing. So long as the universe exists it will contain structures, and some of these will endure, substantially unchanged, longer than will others. No structure endures forever, but its life will be long in proportion as its constitution is such as to contribute to stability. If old structures are destroyed, the conservation of matter and energy demand that new ones should be generated. Nothing could be more in harmony with rational expectation than that the structures which continue to exist--like those of organic species--throughout long periods of time, should be so constituted as to be capable of maintaining such existence. Otherwise, as Darwin correctly indicated, they could not be here. If we regard the process of evolution, thus, from a strictly objective standpoint there is certainly nothing in it anywhere to suggest the notion of purpose.
If, on the other hand, we make a careful study of purpose, we find that it is by no means independent of the Darwinian principles. Although purposes do not explain evolution, evolution can certainly explain purposes.