V.

The same subject continued.

ceives the idea of extension, nor indeed any idea, from SECT. this invariable motion. Even supposing he moves his limbs to and fro, this cannot convey to him that idea. He feels in that case a certain sensation or impression, the parts of which are successive to each other, and may give him the idea of time, but certainly are not disposed in such a manner as is necessary to convey the idea of space or extension.

Since, then, it appears that darkness and motion, with the utter removal of every thing visible and tangible, can never give us the idea of extension without matter, or of a vacuum; the next question is, whether they can convey this idea, when mixed with something visible and tangible?

'Tis commonly allowed by philosophers, that all bodies which discover themselves to the eye, appear as if painted on a plain surface, and that their different degrees of remoteness from ourselves are discovered more by reason than by the senses. When I hold up my hand before me, and spread my fingers, they are separated as perfectly by the blue colour of the firmament, as they could be by any visible object which I could place betwixt them. In order, therefore, to know whether the sight can convey the impression and idea of a vacuum, we must suppose, that amidst an entire darkness, there are luminous bodies presented to us, whose light discovers only these bodies themselves, without giving us any impression of the surrounding objects.

We must form a parallel supposition concerning the objects of our feeling. "Tis not proper to suppose a perfect removal of all tangible objects: we must allow something to be perceived by the feeling; and after an interval and motion of the hand or other organ

PART of sensation, another object of the touch to be met

II.

Of

the ideas of

space and time.

with; and upon leaving that, another; and so on, as often as we please. The question is, whether these intervals do not afford us the idea of extension without body?

To begin with the first case; 'tis evident, that when only two luminous bodies appear to the eye, we can perceive whether they be conjoined or separate; whether they be separated by a great or small distance; and if this distance varies, we can perceive its increase or diminution, with the motion of the bodies. But as the distance is not in this case any thing coloured or visible, it may be thought that there is here a vacuum or pure extension, not only intelligible to the mind, but obvious to the very senses.

This is our natural and most familiar way of thinking, but which we shall learn to correct by a little reflection. We may observe, that when two bodies present themselves, where there was formerly an entire darkness, the only change that is discoverable is in the appearance of these two objects, and that all the rest continues to be as before, a perfect negation of light, and of every coloured or visible object. This is not only true of what may be said to be remote from these bodies, but also of the very distance which is interposed betwixt them; that being nothing but darkness, or the negation of light; without parts, without composition, invariable and indivisible. Now, since this distance causes no perception different from what a blind man receives from his eyes, or what is conveyed to us in the darkest night, it must partake of the same properties; and as blindness and darkness afford us no ideas of extension, 'tis impossible that the dark

and undistinguishable distance betwixt two bodies can SECT. ever produce that idea.

V.

The sole difference betwixt an absolute darkness and The same the subject appearance of two or more visible luminous objects continued. consists, as I said, in the objects themselves, and in the manner they affect our senses. The angles, which the rays of light flowing from them form with each other; the motion that is required in the eye, in its passage from one to the other; and the different parts of the organs which are affected by them; these produce the only perceptions from which we can judge of the distance. But as these perceptions are each of them simple and indivisible, they can never give us the idea of extension.

We may illustrate this by considering the sense of feeling, and the imaginary distance or interval interposed betwixt tangible or solid objects. I suppose two cases, viz. that of a man supported in the air, and moving his limbs to and fro, without meeting any thing tangible; and that of a man, who, feeling something tangible, leaves it, and, after a motion of which he is sensible, perceives another tangible object; and I then ask, wherein consists the difference betwixt these two cases? No one will make any scruple to affirm, that it consists merely in the perceiving those objects, and that the sensation, which arises from the motion, is in both cases the same; and as that sensation is not capable of conveying to us an idea of extension, when unaccompanied with some other perception, it can no more give us that idea, when mixed with the impressions of tangible objects, since that mixture produces no alteration upon it.

But though motion and darkness, either alone or attended tended with tangible and visible objects, convey no

86

II.

не мело Reeping to emmotion of extension Wcoulda It hel

PART idea of a vacuum or extension without matter, yet
are the causes why we falsely imagine we can form
such an idea. For there is a close relation betwixt

Of

the ideas of

space and time.

that motion and darkness, and a real extension, oray
composition of visible and tangible objects.

First, we may observe, that two visible objects, ap-
pearing in the midst of utter darkness, affect the senses
in the same manner, and form the same angle by the
rays
which flow from them, and meet in the eye, as if
the distance betwixt them were filled with visible ob-
jects, that give us a true idea of extension. The sen-
sation of motion is likewise the same, when there is
nothing tangible interposed betwixt two bodies, as
when we feel a compounded body, whose different
parts are placed beyond each other.

Secondly, we find by experience, that two bodies,
which are so placed as to affect the senses in the
same manner with two others, that have a certain ex-
tent of visible objects interposed betwixt them, are ca-
pable of receiving the same extent, without any sensible
impulse or penetration, and without any change on that
angle, under which they appear to the senses. In like
manner, where there is one object, which we cannot
feel after another without an interval, and the perceiv-
ing of that sensation we call motion in our hand or or-
gan of sensation; experience shews us, that 'tis pos-
sible the same object may be felt with the same sensa-
tion 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

V.

the same effects on every natural phenomenon. For as SECT. all qualities, such as heat, cold, light, attraction, &c. diminish in proportion to the distance; there is but The same subject little difference observed, whether this distance be continued. marked 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 phenomenon 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 phenomenon 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 phenomenon may be real, though my explication be chimerical. The falsehood of the one is no consequence of that of the other; though at the same time we may observe,