Psychology however in the strict sense is extremely difficult to define. Those who describe it as the "science of mind," the "phenomenology of consciousness," seem to give it a wider scope than they really mean. The psychologist of the straiter sect tends, on the other hand, to carry us beyond mind and consciousness altogether. His, it has been said, is a psychology without a Psyché. For him Mind, Soul, and Consciousness are only current and convenient names to designate the field, the ground on which the phenomena he observes are supposed to transact themselves. But they must not on any account interfere with the operations; any more than Nature in general may interfere with strictly physical inquiries, or Life and vital force with the theories of biology. The so-called Mind is only to be regarded as a stage on which certain events represent themselves. In this field, or on this stage, there are certain relatively ultimate elements, variously called ideas, presentations, feelings, or states of consciousness. But these elements, though called ideas, must not be supposed more than mechanical or dynamical elements; consciousness is rather their product, a product which presupposes certain operations and relations between them. If we are to be strictly scientific, we must, it is urged, treat the factors of consciousness as not themselves conscious: we must regard them as quasi-objective, or in abstraction from the consciousness which surveys them. The Ego must sink into a mere receptacle or arena of psychic event; its independent meaning or purport is to be ignored, as beside the question.
When this line is once fixed upon, it seems inevitable to go farther. Comte was inclined to treat psychology as falling between two stools: it must, he thought, draw all its content either from physiology on the one hand, or from social factors on the other. The dominant or experimental psychology of the present day seems inclined, without however formulating any very definite statement, to pronounce for the former alternative. It does not indeed adopt the materialistic view that mind is only a function of matter. Its standpoint rather is that the psychical presents itself even to unskilled observation as dependent on (i.e. not independent of) or as concomitant with certain physical or corporeal facts. It adds that the more accurately trained the observer becomes, the more he comes to discover a corporeal aspect even where originally he had not surmised its existence, and to conclude that the two cycles of psychical and physical event never interfere with each other: that soul does not intervene in bodily process, nor body take up and carry on psychical. If it is said that the will moves the limbs, he replies that the will which moves is really certain formerly unnoticed movements of nerve and muscle which are felt or interpreted as a discharge of power. If the ocular impression is said to cause an impression on the mind, he replies that any fact hidden under that phrase refers to a change in the molecules of the brain. He will therefore conclude that for the study of psychical phenomena the physical basis, as it may be called, is all important. Only so can observation really deal with fact capable of description and measurement. Thus psychology, it may be said, tends to become a department of physiology. From another standpoint, biology may be said to receive its completion in psychology. How much either phrase means, however, will depend on the estimate we form of biology. If biology is only the study of mechanical and chemical phenomena on the peculiar field known as an organism, and if that organism is only treated as an environment which may be ignored, then psychology, put on the same level, is not the full science of mind, any more than the other is the full study of life. They both have narrowed their subject to suit the abstract scheme of the laboratory, where the victim of experiment is either altered by mutilation and artificial restrictions, or is dead. If, on the contrary, biology has a substantial unity of its own to which mechanical and chemical considerations are subordinate and instrumental, psychology may even take part with physiology without losing its essential rank. But in that case, we must, as Spinoza said(47), think less mechanically of the animal frame, and recognise (after the example of Schelling) something truly inward (i.e. not merely locally inside the skin) as the supreme phase or characteristic of life. We must, in short, recognise sensibility as the culmination of the physiological and the beginning of the psychological.