in logic, the first and most simple act of the mind, whereby it perceives, or is conscious of its ideas. See LOGIC, Part I. and METAPHYSICS, n° 36—39. 77.

PERCEPTIVE faculty of the human mind.—Concerning this there have been very great controversies. Every one knows that there are sensations arising in our minds; but the question is what it is that perceives them, whether it is a man as a compound being of soul and body, or whether the living percipient is not a mind, or spirit alone, without a body, or else a quality only, resulting from the construction of a body without any distinct or separate spirit annexed thereto. These are difficulties probably never to be demonstrated, and we must at last be content with a probable proof only.

Man is so wonderfully made, that he seems to assign a place to every one of his sensations, and yet reason and experience tell him, that in truth they cannot exist, or be, where he is apt too hastily to judge, or suppose them to be; for as nothing can act where it is not, so the perceptive power of man cannot possibly perceive any thing without or beyond himself. It is generally agreed, that the secondary qualities of body (as they are called) do not exist external to the man, but only the primary ones; though Dr Berkeley attempted to show that they both exist together, and that wherever the colour was, there likewise was the extension. If this could be satisfactorily made to appear, the doctor's system would stand good for the non-existence of every thing but spirit and ideas; but it cannot, and to confine the argument to one sense alone, to wit, sight; that man perceives colour we are sure of, and therefore it must be within him, or he would act where he was not. Now if he perceived extension, that must likewise be within him too, but then he could perceive no extension larger than himself:—but as neither extension nor colour have any place assigned them in the body, surely it is not the body, or any conformation thereof, that perceives. We may then suppose that it is something else, which is joined with the body that is the percipient, which let us name mind or soul; this mind should seem to be one simple uncompounded being, otherwise it could not be conscious that successive perceptions were the affections of the same thing.

Colour, though hastily judged to be without the mind, Berkeley and Malbranche have sufficiently shewn not to be so; and that extension is so, seems also true; because it perceives none of its sensations extended, but only assigns or fixes a place for them, these of colour in particular, external to the man, although in fact they may not be without him; and this place is only determined by an operation of the mind, foggling or supposing distance, from an experimental obstruction to the motion of some members of the body by which the touch is affected as well as the sight, and so both the tangible and visible object concluded, though too precipitately, to be in one and the same place where the obstruction is likewise judged to be; Perceptive, and hence it obtained the supposition or suggestion of distance; and as we have no sensations to which we do not ascribe some distance or place, there must be place or space existing, or it could not be supposed. And therefore as nothing is perceived or suggested but what is supposed in some place, so nothing can exist but what constitutes space, or is in it, and must have some extension.

But then the mind of man surely cannot be extended beyond his body, though it often supposes an extension far beyond; and if the extension imagined was in the mind, and not a mere operation thereof, by way of supposition, it could not guess so much amiss about the extension of objects, which has not been familiar to the other organs of sense, as we often find it does; for it seems to be a vulgar error to entertain a notion of the mind's judging of any distance or magnitudes from any pictures conjectured to be in the fund of the eye, or in itself: in the former case, if there be any picture in the bottom of the eye, it would judge every object in an inverse position to the body, which is contrary to experience; neither does the mind judge of any magnitude according to any such pictures, but of the real external magnitudes; and seldom errs much, unless the objects be very remote.—If the bulk of objects were judged of by the pictures in the eye, a flea or mite must judge every object very small to what a man does, because the picture will be diminished nearly as the eye is less: indeed these insects may see distinctly smaller things than man, because the objects may be brought nearer their small eyes, without throwing the focus of the rays beyond the retina, as the same distance of object would do in a larger eye, and prevent distinct vision; and it is highly probable, that these small insects cannot see objects at a great distance, unless they are much larger than what a man can see at the like distance; but then what they do see they judge to be of the same bigness that a man does; and so must every creature, let its eyes be of what dimension or number you please. It is a vague notion opticians have, who imagine that one, like a microscope lens, will magnify the picture on the retina, whereas just the contrary takes place; for when the eye is used alone, without such a lens, the shorter focus of the eye forms the picture, and the longer is at the object; but when a lens is used by way of a microscope, the object is in the shorter, and the picture at the longer focus, just contrary to the method of common vision.

So, again, if the mind was conscious of a picture in the eye, it would perceive as many objects the creature had eyes; whereas it judges of no more, let the number of eyes be as they will, than it does by the help of any other of the senses.

From all which we may conclude, that figure, extension, and motion, are not perceptible objects; but that sensations alone are such, the former being only imagined, by an operation of the mind, to exist external to it; and that if they did not so exist, the mind could not imagine any extension, figure, and motion; for there never is found any of them perceivable by it, nor any figure or motion attending a simple sensation. Indeed it is too commonly thought, that there is a shape perceived with colour, or a coloured shape; but no object appears of one simple colour to a fixed eye, but every part of the object exhibits a different degree of colour; and these degrees separate sensations, to which the mind ascribes a place, though, in fact, the colour is not in the place so judged of; but something else that gives reliance to the actions of the mind on the body; and from hence it supposes there must be something existing there which gives rise to the colour perceived by it.—It is impossible the mind should perceive the images of things within itself, unless it was equally extended with the things themselves; and if not, how can it be thought that an ideal world can exist within the mind, as some philosophers have conjectured? Surely it cannot be; but it must be only imagination that directs as to the external existence of real things. We cannot properly be said to imagine what does not or has not really existed; for let a blind man try if he can imagine colour, or a deaf man sound, and he will find himself at a loss. Father Malbranche indeed tells us, that a man may have an idea of a golden mountain that never existed; and a man may recollect the figure of a mountain which he has formerly imagined, and remember the colour of gold which he lately had a perception of, and suppose it possible they may be connected, and call this operation of his mind an idea if he pleases; but, after all his efforts, if he should happen to think of a mountain as large as Shooter's hill, he will hardly allow it to be contained in his mind.