It comes in as _an explanatory concept_, like the older concept of association (p. 146), to account for, to rationalise, the phenomena that are conscious. We have ourselves been satisfied with description and correlation, and we have therefore confined ourselves to mental and nervous processes which are in principle observable; though we have often enough been obliged to say that the facts, in this or that chapter of psychology or neurology, are few or wanting. There is, however, in many minds, a craving for ‘explanation’; and it must be admitted that such a craving is natural enough; for it shows in every phase of primitive thought, and may be traced throughout the history of science. Think, for instance, of the potency of explanation by ‘cause and effect’!—though when we examine a case of cause and effect we never, in fact, find anything more than correlation. There are many psychologists, then, who cannot be satisfied with description and correlation; they must refer the direction of thought to a ‘subconscious disposition,’ and explain the connections of ideas by ‘subconscious tendencies,’ and so on. They have recourse to the subconscious for purposes of explanation. We must urge two objections against this mode of psychologising. In the first place, _the construction of a subconscious is unnecessary_. Science is not called upon to ‘explain’ anything; description and correlation are the modern—and more modest—representatives of the ‘explanation’ that an older science looked for and professed to find. Secondly, _the introduction of a subconscious is dangerous_. It is a matter of inference from the conscious; but who shall draw the line, in such a case, between legitimate and illegitimate inference? When from the course of the mental stream and the interplay of mental processes we infer the existence of associative and determining tendencies in the nervous system, our argument is safeguarded. No man, it is true, has seen those tendencies in course; but the inference to them is checked and controlled by the whole vast body of fact and method that makes up modern physiology. Things stand very differently with the subconscious. Here the inference must, it is plain, go beyond the conscious, since its aim is to explain the conscious; yet the conscious facts are all the facts we have; when once we have embarked on the subconscious, there are no more facts to steer by. Henceforth everything depends upon individual preference; and we may have many theories of the subconscious, widely different and equally plausible. The danger is that an erroneous theory of the subconscious distort our view of the conscious.