more in prosecution of my first maxim, that we must in the end
rest contented with experience, than for want of something specious and
plausible, which I might have displayed on that subject. It would have
been easy to have made an imaginary dissection of the brain, and have
shewn, why upon our conception of any idea, the animal spirits run
into all the contiguous traces, and rouze up the other ideas, that are
related to it. But though I have neglected any advantage, which I might
have drawn from this topic in explaining the relations of ideas, I am
afraid I must here have recourse to it, in order to account for the
mistakes that arise from these relations. I shall therefore observe,
that as the mind is endowed with a power of exciting any idea it
pleases; whenever it dispatches the spirits into that region of the
brain, in which the idea is placed; these spirits always excite the
idea, when they run precisely into the proper traces, and rummage that
cell, which belongs to the idea. But as their motion is seldom direct,
and naturally turns a little to the one side or the other; for this
reason the animal spirits, falling into the contiguous traces, present
other related ideas in lieu of that, which the mind desired at first to
survey. This change we are not always sensible of; but continuing
still the same train of thought, make use of the related idea, which is
presented to us, and employ it in our reasoning, as if it were the same
with what we demanded. This is the cause of many mistakes and sophisms
in philosophy; as will naturally be imagined, and as it would be easy to
show, if there was occasion.

Of the three relations above-mentioned that of resemblance is the most
fertile source of error; and indeed there are few mistakes in reasoning,
which do not borrow largely from that origin. Resembling ideas are not
only related together, but the actions of the mind, which we employ
in considering them, are so little different, that we are not able to
distinguish them. This last circumstance is of great consequence, and we
may in general observe, that wherever the actions of the mind in forming
any two ideas are the same or resembling, we are very apt to confound
these ideas, and take the one for the other. Of this we shall see many
instances in the progress of this treatise. But though resemblance be
the relation, which most readily produces a mistake in ideas, yet
the others of causation and contiguity may also concur in