When I received the relations of resemblance, contiguity and causation,
as principles of union among ideas, without examining into their causes,
it was 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