Psychology treats, roughly speaking, of mentality; it comprises a study of the establishment of sets of inner relations, (_i.e._, associations of thought, relations of ideas, relations of sequences, the powers of remembrance, of discrimination and identification,) with sets of external relations, namely, the actual existences of which the inner relations are the representatives. The establishment of such inner relations corresponding to outer relations and their widening growth, must have a marked influence upon human conduct so that it may very well be separated for convenience of study from the earlier forms of organic conduct, in which such action is little recognisable. But how to form the connective law is the difficulty. Moreover, it is one thing to establish the fact of evolution, and another thing to explain it. We ourselves admit the fact, indeed, but search in vain for the explanation. Are we to look for the origin of Pleasure and Pain in those laws of the moving equilibrium which necessitate the generation of internal forces equal and in opposition to external inimical forces? If so, Pleasure and Pain must be regarded as forces--as factors--in the organism, and we must regard the subjective as generated by external physical factors operating upon internal physical factors, and we must regard these subjective factors not merely as concomitant, but as producing physical effects by way of reaction. So far as it goes there may be a physical view of purposed conduct, and so far as it goes there may be a psychological view, but between the two the biological view is a mere disorderly mixture, borrowing its terms first on one hand and then on the other, and assigning its determining causes first to the physical moving equilibrium theory, and then again to the anticipation of Pleasure and Pain. But the biological law which should co-ordinate these two sets of laws is not formulated, and hence we find more or less gliding, or more or less sudden transition from one set of terms or laws to the other, a defect which is concealed in some measure by the formal divisions of the chapters. But if the course of thought is carefully followed it is found that the actual treatment does not properly fit in. There is an unmistakeable transition from the purely physical set of factors to the purely subjective, and there is no co-ordinated biological law at all. The chapter is a transitional one, it is true, but only in the sense of gradually leaving off the employment of one set of terms, and the gradual employment of another set of terms in the treatment of the same phenomena. Mr. Spencer argues well in Chapter V. as to the concomitance of pleasure-giving acts with life-sustaining acts, and of pain-giving acts with decrease of life; but which is prior in the chain of causation? Or, to repeat the old difficulty, is the subjective factor present in the line of causation at all? Is it merely a concomitant of the physical line of events?