already excluded from the imagination: Nor yet are we to
conclude, that without it the mind cannot join two ideas; for nothing
is more free than that faculty: but we are only to regard it as a
gentle force, which commonly prevails, and is the cause why, among other
things, languages so nearly correspond to each other; nature in a manner
pointing out to every one those simple ideas, which are most proper to
be united in a complex one. The qualities, from which this association
arises, and by which the mind is after this manner conveyed from one
idea to another, are three, viz. RESEMBLANCE, CONTIGUITY in time or
place, and CAUSE and EFFECT.

I believe it will not be very necessary to prove, that these qualities
produce an association among ideas, and upon the appearance of one idea
naturally introduce another. It is plain, that in the course of our
thinking, and in the constant revolution of our ideas, our imagination
runs easily from one idea to any other that resembles it, and that this
quality alone is to the fancy a sufficient bond and association. It
is likewise evident that as the senses, in changing their objects,
are necessitated to change them regularly, and take them as they lie
CONTIGUOUS to each other, the imagination must by long custom acquire
the same method of thinking, and run along the parts of space and time
in conceiving its objects. As to the connexion, that is made by the
relation of cause and effect, we shall have occasion afterwards to
examine it to the bottom, and therefore shall not at present insist
upon it. It is sufficient to observe, that there is no relation, which
produces a stronger connexion in the fancy, and makes one idea more
readily recall another, than the relation of cause and effect betwixt
their objects.

That we may understand the full extent of these relations, we must
consider, that two objects are connected together in the imagination,
not only when the one is immediately resembling, contiguous to, or the
cause of the other, but also when there is interposed betwixt them a
third object, which bears to both of them any of these relations. This
may be carried on to a great length; though at the same time we may
observe, that each remove considerably weakens the relation. Cousins in
the fourth degree are connected by causation, if I may be allowed to
use that term; but not so closely as brothers, much less as child and
parent. In general we may observe, that all the relations o