We shall now proceed to enquire how we may reduce these principles to a lesser number, and find among the causes something common, on which their influence depends.
In order to this we must reflect on certain properties of human nature, which though they have a mighty influence on every operation both of the understanding and passions, are not commonly much insisted on 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 considerable time; nor can it by its utmost efforts ever arrive at such a 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, united by these relations, naturally follows it, and enters with more facility by means of that introduction.
The second property I shall observe in the human mind is a like association of impressions.