PERCEPTION is that power or act of the mind by which it holds communication with the external world. It is distinguished from conception by the circumstance that its objects are in every instance supposed to have an actual existence. We may conceive things that have no reality, but we aro never said to perceive such things. Per ception differs from consciousness in that it takes cognisance only of objects without the mind. We perceive a man, a horse, a tree ; when we think or feel, we are conscious of our thoughts and emotions. It is further supposed in perception that the objects of it are present. We can remember former objects of perception, but we do not perceive them again until they are once more present. Besides the sense which has been explained, the term perception is sometimes analogically employed in common speech in reference to truths the evidence of which is certain. Thus we may perceive the truth of a mathematical proposition. But Mr. Hume is perhaps the only writer of eminence who designedly applies the word in a metaphysical disquisition with a meaning different from that which has been here assigned to it. By him it is applied indiscriminately to all the operations and states of the mind ; passions being designated perceptions, and the acts of memory and imagination converted into so many acts of perception. Such latitude of phraseology oonfounds under one general name things essentially distinct, and tends to introduce vagueness and inaccuracy into a department of philosophical investigation where definiteness and precision are peculiarly indispensable.
Tho distinction between things perceived (aloOnrci) and things con cei red (vonrh)was familiar to the Greek philosophers and to their Latin expositors, of whom Cicero expresses the former class of things by the phrases "gum sunt," " qure cerni tangive possunt ;" and the latter by "qua) tangi dsmonstrarive non possunt, cerni tamen animo atque intelligi possunt," and gives examples of each. (` Top.' v.) The perceptive faculty is exercised through the instrumentality of the senses. We see by means of the eye, and hear by means of the ear, and so in reference to the other senses. An individual in whom these organs are wanting or defective, will either not perceive at all, or perceive imperfectly. In order to perception it is requisite that an impression should be made on the organ of sense, either by the direct application of the object, or through some medium that communicates with the object and the organ. Thus an immediate application is
necessary with regard to the senses of taste and touch ; but only an intermediate one with regard to those of sight, bearing, and smell. The impression made on the organs of sense affects the nerves, and is by them conveyed to the brain. The necessity of this communication is ascertained by observation. If the nerve appropriated to any organ be cut or tied hard, no perception takes place; and the same result is noticed in certain disordered conditions of the brain, even though the organs of sense and the nerves perform their respective functions. When however the conditions that have been specified are complied with, perception ensues.
Various theories have been formed to explain the functions of the nerves and brain in connection with perception. It was Imagined by the ancients that the nervous fibres are tubular, and filled with a subtlle vapour named animal spirits ; that the brain is a gland by which this ethereal fluid is secreted ; and that by means of it the nerves perform their office. (Reid, Essay' ii., ch. iii.) Des Cartes, who adopted this hypothesis, has described with great minuteness how all mental operations and movements are accomplished through the agenoy referred to. Dr. Briggs, Newton's instructor in anatomy, was the first who proposed a new doctrine on this point. He maintained that the nerves operate by vibrations, like musical chords, and thus conduct impressions to the brain. Newton himself (' Opt.,' qu. 23) appears to have been inclined to a notion of this kind, and the sugges tions relating to it thrown out by him as a query were afterwards amplified and defended by Hartley. The latter supposed that" external objects impressed on the senses occasion, first in the nerves on which they are impressed and then in the brain, vibrations of the small and, as one may say, infinitesimal medullary particles ;" and that these vibrations " arc excited, propagated, and kept up partly by the ether, that is, by a very subtile elastic fluid : partly by the uniformity, continuity, softness, and active powers of the medullary substance of the brain, spinal marrow, and nerves." ('Observations on Man: part 1., prop. 4, 5.) Both Dee Cartes and Hartley believed that by the action of the nerves In the manner described by them, images of external objects were formed in the brain.