be felt
with the same sensation of motion, along with the interposed impression
of solid and tangible objects, attending the sensation. That is, in
other words, an invisible and intangible distance may be converted into
a visible and tangible one, without any change on the distant objects.

Thirdly, We may observe, as another relation betwixt these two kinds
of distance, that they have nearly the same effects on every natural
phaenomenon. For as all qualities, such as heat, cold, light,
attraction, &c. diminish in proportion to the distance; there is but
little difference observed, whether this distance be marled out by
compounded and sensible objects, or be known only by the manner, in
which the distant objects affect the senses.

Here then are three relations betwixt that distance, which conveys the
idea of extension, and that other, which is not filled with any coloured
or solid object. The distant objects affect the senses in the same
manner, whether separated by the one distance or the other; the second
species of distance is found capable of receiving the first; and they
both equally diminish the force of every quality.

These relations betwixt the two kinds of distance will afford us an easy
reason, why the one has so often been taken for the other, and why we
imagine we have an idea of extension without the idea of any object
either of the sight or feeling. For we may establish it as a general
maxim in this science of human nature, that wherever there is a close
relation betwixt two ideas, the mind is very apt to mistake them, and
in all its discourses and reasonings to use the one for the other. This
phaenomenon occurs on so many occasions, and is of such consequence,
that I cannot forbear stopping a moment to examine its causes. I shall
only premise, that we must distinguish exactly betwixt the phaenomenon
itself, and the causes, which I shall assign for it; and must not
imagine from any uncertainty in the latter, that the former is also
uncertain. The phaenomenon may be real, though my explication be
chimerical. The falshood of the one is no consequence of that of the
other; though at the same time we may observe, that it is very natural
for us to draw such a consequence; which is an evident instance of that
very principle, which I endeavour to explain.

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