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.