19. Though it be too obvious to escape observation, that different
ideas are connected together; I do not find that any philosopher has
attempted to enumerate or class all the principles of association; a
subject, however, that seems worthy of curiosity. To me, there
appear to be only three principles of connexion among ideas, namely,
Resemblance, Contiguity in time or place, and Cause or Effect.
That these principles serve to connect ideas will not, I believe, be
much doubted. A picture naturally leads our thoughts to the original:*
the mention of one apartment in a building naturally introduces an
enquiry or discourse concerning the others:*(2) and if we think of a
wound, we can scarcely forbear reflecting on the pain which follows
it.*(3) But that this enumeration is complete, and that there are no
other principles of association except these, may be difficult to
prove to the satisfaction of the reader, or even to a man’s own
satisfaction. All we can do, in such cases, is to run over several
instances, and examine carefully the principle which binds the
different thoughts to each other, never stopping till we render the
principle as general as possible.*(4) The more instances we examine,
and the more care we employ, the more assurance shall we acquire, that
the enumeration, which we form from the whole, is complete and entire.