The conditions of the evolution of intelligence as influential on behaviour are. on the physiological side, the increased differentiation, size, and complexity of the secondary centres, and on the psychological side the opportunities thus afforded for the coalescence of the situation revived or re-presented into an increasingly complex body of practical experience, the items of which have increasingly intricate cross-reference in terms of meaning with more varied phases of satisfaction value. All this is classified under the head of perceptual process.
But upon this, at some stage of evolution, there began to be superimposed the higher ideational and conceptual process. The physiological conditions are a differentiation of new tertiary centres in the nervous system. The psychological conditions are those modes of mental activity known as comparison and generalisation, analysis and synthesis. Hence arise those ideal constructions, whose genesis we seek, which may be regarded as mental products crystallised out of the magma of naive perceptual experience. Not that this analogy serves to do more than illustrate, and that inadequately, the manner of their genesis. They are, it must be remembered, the concomitants of physiological configurations undergoing organic changes. The way in which these exercise a guiding influence on the functioning of the lower centres must be a specifically physiological mode of interaction; but, as such, it is in line with all that biology teaches as to the functions of a nervous system.
Now it may be frankly admitted that we do not at present know all the conditions, physiological and psychological, under which the specific mental products of perceptual and ideational process originate, and under which the control of bodily activities arises. But we have made some advance towards such knowledge in terms of a naturalistic interpretation. Granted that this is carried very much further, so as to enable us to afford an explanation of mental evolution and development in some degree adequate to the case, even then we shall be told that we have not reached the heart of the matter. It will be said that we have done no more than to describe the conditions under which certain mental products occur, but that we have rendered no account of the reason why, under these conditions, they should occur, or why they are of this or that particular nature. I would, however, again insist on the fact that naturalism does not, or at any rate should not, profess to give answers to any such questions. When naturalism has traced the antecedent conditions of the origin of any product of nature it has performed its task to the full. Why the product is what it is and not something else neither naturalism nor science can say. The facts have to be accepted as given factsó and there's an end on it.
That is just where naturalism fails fully to satisfy the needs of the inquiring human mind. A further question will arise and press for an answer. Even supposing that the many gaps in assured knowledge of the conditions of the sequence implied in mental evolution shall some day be adequately bridged, is such an account, or any such account, philosophically satisfactory? We have an ascending evolutionary curve; can we, who are points in the course of its sweep, give a satisfactory account of it by looking only backward at the foregoing conditions and not at all forward to the end to which it tends? Granting that the sequence is of such a kind as to make a rational appeal to the reason which is, here and now, its final outcome, may we say that in this outcome reason is becoming conscious of itself as partaking of the nature of the underlying cause?
Or, to return to the ethical conceptions with which we started in this section, must not similar questions suggest themselves? No doubt ethics may be treated from a frankly scientific point of view. We find, as a matter of fact, that men and womenósome of them civilised like ourselves, some of them with very different social notions from oursódo form ideals of one kind or another, though we may often think them very wrong-headed. These ideals may be classified, the nature of their sequence may be described, and generalisations may be reached as to their mode of development. In all of this the treatment of ethics is proceeding on scientific lines. But the question arises, Why does a man have ideals at all? We are perhaps told that they are the natural outcome of his character and the circumstances of his life and upbringing. No doubt they are. I would not for a moment deny that in the formation of every ideal there is a chain of antecedents, the links of which we might, but often cannot, unravel. I do not deny that every man's character and personality is a synthesis of elements, the stages of which might be traced if only we had adequate insight and knowledge. But it seems to me that of this synthesis there is a cause, which for metaphysics is the will of that individual. And then the further question arises, Why, having such ideals, does a man act on them? From the strictly scientific point of view the bare fact must be accepted. The act is the sequent of the influence of the foregoing physiological configuration, concomitant with which is an antecedent psychological disposition. True enough. But is this an adequate and satisfactory interpretation of human conduct? Can we rule out purpose and end and the desire for their attainment as real causes of man's endeavour? I for one think not. Conduct and history lose their meaning if we do. May we not say that the realities of practical and intellectual life and of moral endeavour are the ends for which men strive?