A Treatise of Human Nature - Volume 2

and that it is not by a different principle that each different cause is adapted to its passion. We shall now proceed to inquire how we may reduce these principles to a smaller number, and find something common among the causes on which their influence depends.

S E C T. IV.

In order to do this, we must reflect on certain properties of human nature, which, though they have a great influence on every operation of both the understanding and the passions, are not commonly much emphasized by philosophers. The first of these is the association of ideas, which I have so often observed and explained. It is impossible for the mind to fix itself steadily upon one idea for any significant length of time; nor can it, by its utmost efforts, ever arrive at such constancy. But however changeable our thoughts may be, they are not entirely without rule and method in their changes. The rule by which they proceed is to pass from one object to what is resembling, contiguous to, or produced by it. When one idea is present to the imagination, any other idea united by these relations naturally follows it and enters with more ease by means of that introduction.

The second property I shall observe in the human mind is a similar association of impressions.