I shall conclude this subject of extension with a paradox, which will easily be explained from the foregoing reasoning.  This paradox is, that if you are pleased to give to the in-visible and intangible distance, or in other words, to the capacity of becoming a visible and tangible distance, the name of a vacuum, extension and matter are the same, and yet there is a vacuum.  If you will not give it that name, motion is possible in a plenum, without any impulse in infinitum, without returning in a circle, and without penetration.  But however we may express ourselves, we must always confess, that we have no idea of any real extension without filling it with sensible objects, and conceiving its parts as visible or tangible.

As to the doctrine, that time is nothing but the manner, in which some real objects exist; we may observe, that it is liable to the same objections as the similar doctrine with regard to extension.  If it be a sufficient proof, that we have the idea of a vacuum, because we dispute and reason concerning it; we must for the same reason have the idea of time without any changeable existence; since there is no subject of dispute more frequent and common.  But that we really have no such idea, is certain.  For whence should it be derived?  Does it arise from an impression of sensation or of reflection?  Point it out distinctly to us, that we may know its nature and qualities.  But if you cannot point out any such impression, you may be certain you are mistaken, when you imagine you have any such idea.

But though it be impossible to shew the impression, from which the idea of time without a changeable existence is derived; yet we can easily point out those appearances, which make us fancy we have that idea.  For we may observe, that there is a continual succession of perceptions in our mind; so that the idea of time being for ever present with us; when we consider a stedfast object at five-a-clock, and regard the same at six; we are apt to apply to it that idea in the same manner as if every moment were distinguished by a different position, or an alteration of the object.  The first and second appearances of the object, being compared with the succession of our perceptions, seem equally removed as if the object had really changed.  To which we may add, what experience shews us, that the object was susceptible of such a number of changes betwixt these appearances; as also that the unchangeable or rather fictitious duration has the same effect upon every quality, by encreasing or diminishing it, as that succession, which is obvious to the senses.  From these three relations we are apt to confound our ideas, and imagine we can form the idea of a time and duration, without any change or succession.

SECT.  VI.  OF THE IDEA OF EXISTENCE, AND OF EXTERNAL EXISTENCE.

It may not be amiss, before we leave this subject, to explain the ideas of existence and of external existence; which have their difficulties, as well as the ideas of space and time.  By this means we shall be the better prepared for the examination of knowledge and probability, when we understand perfectly all those particular ideas, which may enter into our reasoning.