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 the same influence. We
might produce the figures of poets and orators, as sufficient proofs of
this, were it as usual, as it is reasonable, in metaphysical subjects to
draw our arguments from that quarter. But lest metaphysicians should
esteem this below their dignity, I shall borrow a proof from an
observation, which may be made on most of their own discourses, viz. that
it is usual for men to use words for ideas, and to talk instead of
thinking in their reasonings. We use words for ideas, because they are
commonly so closely connected that the mind easily mistakes them. And
this likewise is the reason, why we substitute the idea of a distance,
which is not considered either as visible or tangible, in the room of
extension, which is nothing but a composition of visible or tangible
points disposed in a certain order. In causing this mistake there concur
both the relations of causation and resemblance. As the first species of
distance is found to be convertible into the second, it is in this respect
a kind of cause; and the similarity of their manner of affecting the
senses, and diminishing every quality, forms the relation of resemblance.

After this chain of reasoning and explication of my principles, I am now
prepared to answer all the objections that have been offered, whether
derived from metaphysics or mechanics. The frequent disputes concerning a
vacuum, or extension without matter prove not the reality of the idea,