.

The principles of union among ideas, I have reduced to three general
ones, and have asserted, that the idea or impression of any object
naturally introduces the idea of any other object, that is resembling,
contiguous to, or connected with it. These principles I allow to be
neither the infallible nor the sole causes of an union among ideas.
They are not the infallible causes. For one may fix his attention during
Sometime on any one object without looking farther. They are not the
sole causes. For the thought has evidently a very irregular motion in
running along its objects, and may leap from the heavens to the earth,
from one end of the creation to the other, without any certain method
or order. But though I allow this weakness in these three relations, and
this irregularity in the imagination; yet I assert that the only general
principles, which associate ideas, are resemblance, contiguity and
causation.

There is indeed a principle of union among ideas, which at first sight
may be esteemed different from any of these, but will be found at
the bottom to depend on the same origin. When every individual of any
species of objects is found by experience to be constantly united with
an individual of another species, the appearance of any new individual
of either species naturally conveys the thought to its usual attendant.
Thus because such a particular idea is commonly annexed to such a
particular word, nothing is required but the hearing of that word to
produce the correspondent idea; and it will scarce be possible for the
mind, by its utmost efforts, to prevent that transition. In this case it
is not absolutely necessary, that upon hearing such a particular sound
we should reflect on any past experience, and consider what idea
has been usually connected with the sound. The imagination of itself
supplies the place of this reflection, and is so accustomed to pass from
the word to the idea, that it interposes not a moment's delay betwixt
the hearing of the one, and the conception of the other.

But though I acknowledge this to be a true principle of association
among ideas, I assert it to be the very same with that betwixt the ideas
of cause and effects and to be an essential part in all our reasonings
from that relation. We have no other notion of cause and effect, but
that of certain objects, which have been always conjoined together,
and which in all past instances have been found inseparable. We cannot
penetrate