[Footnote 31: In metaphysics we reason, not on facts, but most often on conceptions. Now just as facts are precise so conceptions are vague in outline. Facts are like crystallised bodies, ideas like liquids and gases. We think we have an idea, and it changes form without our perceiving it. We fancy we recognise one idea, and it is but another, which differs slightly from the preceding one. By means of distinctions we ought to struggle against this flowing away and flight of ideas.] [Footnote 32: I think I have come across in ARISTOTLE the ingenious idea that the enfeeblement of the consciousness and its disorder may be due to the enfeeblement and disorder of the object. It is a theory which is by no means improbable.] CHAPTER IX DEFINITIONS OF PSYCHOLOGY Let us resume the study of the preceding ideas in another form. Since, moreover, to define mind is at the same time to define psychology, let us seek for the truth which we can glean from the definitions of this science. Our object is not to discover an exact definition, but to make use of those already existing. To define psychology is to describe the features of the domain over which this science holds sway, and at the same time to indicate the boundaries which separate it from its neighbours. At first sight this is an affair of geometric survey, presenting no kind of difficulty; for psychology does not merge by insensible transitions into the neighbouring sciences, as physics does with chemistry, for example, or chemistry with biology. To all the sciences of external nature psychology offers the violent opposition of the moral to the physical world. It cannot be put in line with the physical sciences. It occupies, on the contrary, a position apart. It is the starting point, the most abstract and simple of the moral sciences; and it bears the same relation to them that mechanics does to the physical. All this is doubtless true; and yet a very great difficulty has been experienced in condensing into a clear definition the essence of psychology. This is proved by the multiplicity of definitions attempted. They are so many because none of them has proved completely satisfactory. Their abundance shows their insufficiency. I will try to introduce a little order into these attempts, and propose to distribute the definitions of psychology into the following categories:-- 1. The definition by substance; the metaphysical definition _par excellence_. 2. The definition by enumeration. 3. " " method. 4. " " degree of certainty. 5. " " content. 6. " " point of view. 7. " " the peculiar nature of mental laws. We will rapidly run through this series of efforts at definition, and shall criticise and reject nearly the whole of them; for the last alone seems exact--that is to say, in harmony with the ideas laid down above.