For why does the reflex act appear to us to be purposive? Why does it seem to us as if it were guided by foresight? Precisely because it is determinate; precisely because, as in human volition, the end is implicitly contained in the antecedent conditions. Long has been the controversy between the upholders of determinism and those who contend for purpose in nature. No agreement can be reached until both parties realise that they are looking at different sides of the same shield. Determinism, whether in external nature or in human life, is the expression of purpose; purpose is that which finds expression in determinate sequence. The criterion of determinism is that all the conditions of the sequent event are contained within the antecedent configuration; and this, according to naturalism itself, is just that which analysis discloses as the characteristic feature of human volition and purpose. Either, therefore, purpose has no existence at all, or it is that of which all determinate sequence is the phenomenal expression.
My aim has been to show that a belief in purpose as the causal reality of which nature is an expression is not inconsistent with a full and whole-hearted acceptance of the explanations of naturalism, within their appropriate sphere. At the outset I contrasted two modes of interpretation—that which works outwards from human life as a centre, and explains the world in terms of purpose analogous to the purposes of man, and that which works inwards from external mechanism to that of the human brain, with its associated mental states, and explains the universe in terms of natural law. At the close I reach the conclusion that it is not impossible to bring these views into harmony, if we can accept the postulate that determining purpose is the reality which underlies the determinate course of phenomena. Each is supplementary to the other. Neither can be substituted for the other within the same universe of discourse. We may not interpolate purpose in a scientific curve of antecedences and sequences, just to fill in the gaps due to our present ignorance of physical and psychological conditions. Nor may we interpolate inorganic or organic mechanism within the curve of purpose, save as a manifestation of that causal existence which gives uniformity and continuity to the course of events.
Except for passing reference near the beginning of this discussion I have been obliged to leave on one side that which I spoke of as the aesthetic appeal. A separate essay would be needed for its further consideration. That it only attains to its highest value and worth when it is associated with an interpretation of nature in terms of purpose is my firm conviction. The aesthetic appeal, like the appeal of purpose, is essentially projected from the thoughts, feelings, and emotions of man on to the plane of.