| 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 extent of visible objects interposed betwixt them, are capable 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 perceiving of that sensation we call motion in our hand or organ of sensation; experience shews us, that it is possible the same object may 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 more in prosecution of my first maxim, that we must in the end rest contented with experience, than for want of something specious and plausible, which I might have displayed on that subject. |
| It would have been easy to have made an imaginary dissection of the brain, and have shewn, why upon our conception of any idea, the animal spirits run into all the contiguous traces, and rouze up the other ideas, that are related to it. |
| But though I have neglected any advantage, which I might have drawn from this topic in explaining the relations of ideas, I am afraid I must here have recourse to it, in order to account for the mistakes that arise from these relations. |
| I shall therefore observe, that as the mind is endowed with a power of exciting any idea it pleases; whenever it dispatches the spirits into that region of the brain, in which the idea is placed; these spirits always excite the idea, when they run precisely into the proper traces, and rummage that cell, which belongs to the idea. |
| But as their motion is seldom direct, and naturally turns a little to the one side or the other; for this reason the animal spirits, falling into the contiguous traces, present other related ideas in lieu of that, which the mind desired at first to survey. |
| This change we are not always sensible of; but continuing still the same train of thought, make use of the related idea, which is presented to us, and employ it in our reasoning, as if it were the same with what we demanded. |
| This is the cause of many mistakes and sophisms in philosophy; as will naturally be imagined, and as it would be easy to show, if there was occasion. |