What may one lawfully mean by saying that Psychology ought to be treated after the fashion of a 'natural science'? I think that I can state what I mean; and I even hope that I can enlist the sympathy of men like Professor Ladd in the cause, when once the argument is fairly set forth.
What is a natural science, to begin with? It is a mere fragment of truth broken out from the whole mass of it for the sake of practical effectiveness exclusively. Divide et impera. Every special science, in order to get at its own particulars at all, must make a number of convenient assumptions and decline to be responsible for questions which the human mind will continue to ask about them. Thus physics assumes a material world, but never tries to show how our experience of such a world is 'possible.' It assumes the inter-action of bodies, and the completion by them of continuous changes, without pretending to know how such results can be. Between the things thus assumed, now, the various sciences find definite 'laws' of sequence; and so are enabled to furnish general Philosophy with materials properly shaped and simplified for her ulterior tasks. If, therefore, psychology is ever to conform to the type of the other natural sciences, it must also renounce certain ultimate solutions, and place itself on the usual common-sense basis by uncritically begging such data as the existence of a physical world, of states of mind, and of the fact that these latter take cognizance of other things. What the 'physical world' may be in itself, how 'states of mind' can exist at all, and exactly what 'taking cognizance' may imply, are inevitable further questions; but they are questions of the kind for which general philosophy, not natural science, is held responsible.
Now if there is any natural science in possession of a subject-matter well set off and contrasted with all others, it is psychology. However much our self-consciousness, our freedom, our ability to conceive universals, or what not, may ally us with the Infinite and Absolute, there is yet an aspect of our being, even of our mental being, which falls wholly within the sphere of natural history. As constituting the inner life of individual persons who are born and die, our conscious states are temporal
