Perception This word refers to our reception of knowledge through the senses, an operation which to the common understanding seems simple enough; but, viewed philosophically, is attended with much difficulty. Perception, considered as a source of knowledge, refers exclusively to the outer, or the object world — the world of extended matter and its properties. The names for the act of knowing one's own mind — the feelings and thoughts of the individual are self-consciousness and self-introspection. The word "consciousness" is sometimes improperly limited to this signification. Locke used the term "reflection" for the same meaning; but this is ambiguous, and is now disused. All our knowledge is thus said (by those that deny innate ideas) to spring from two sources — perception and self- consciousness.
Sir William Hamilton (Intel. Pow. essay i, ch. i) notices the following meanings of perception, as applied to different faculties, acts, and objects
1. Perceptio, in its primary philosophical signification, as in the mouths of Cicero and Quintilian, is vaguely equivalent to comprehension, notion, cognition in general.
2. An apprehension, a becoming aware of, consciousness. Perception the Cartesians really identified with idea, and allowed them only a logical distinction; the same representative act being called idea, inasmuch as we regard it as a representation; and perception, inasmuch as we regard it as a consciousness of such representation.
3. Perception is limited to the apprehension of sense alone. This limitation was first formally imposed by Reid, and thereafter by Kant.
4. A still more restricted meaning, through the authority of Reid, is perception (proper), in contrast to sensation (proper). He defines sensitive perception simply as that act of consciousness whereby we apprehend in our body,
(a) certain special affections, whereof, as an animated organism, it is contingently susceptible; and
(b) those general relations of extension under which, as a material organism, it necessarily exists.
Of these perceptions, the former, which is thus conversant about a subject object, is sensation proper; the latter, which is thus conversant about an object-object, is perception proper.
Two great disputes connect themselves with perception, both raised into their full prominence in the philosophical world by bishop Berkeley. The first is the origin of our judgments of the distances and real magnitudes of visible bodies. In opposition to the common opinion on this subject, Berkeley maintained that these were learned by experience, and not known by the mere act of vision. The second question relates to the grounds we have for asserting the existence of an external and material world, which, in the view of Berkeley, was bound up with the other. Inasmuch as perception is a mental act, and knowledge is something contained in a mind, what reason have we for believing in the existence of objects apart from our minds? or what is the mode of existence of the so-called external world? The following sentences show in what manner Berkeley opened up the question: "That neither our thoughts nor passions nor ideas, formed by the imagination, exist without the mind, is what everybody will allow; and it seems no less evident that the various sensations or ideas imprinted on the sense, however blended or combined together (i.e. whatever objects they compose), cannot exist otherwise than in a mind perceiving them. I think an intuitive knowledge may be obtained of this by any one that shall attend to what is meant by the term exist when applied to sensible things. The table I write on I say exists — i.e. I see and feel it; and if I were out of my study, I should say it existed, meaning thereby that if I was in my study I might perceive it, or that some other spirit actually does perceive it. There was an odor i.e. it was smelled; there was a sound — that is to say, it was heard; a color or figure, and it was perceived by sight or touch. This is all I can understand by these and the like expressions. For as to what is said of the absolute existence of unthinking things, without any relation to their being perceived, that seems perfectly unintelligible. Their esse is percipi, nor is it possible they should have any existence out of the minds or thinking things which perceive them." SEE BERKELEY. This doctrine of Berkeley, amounting, it was said, to a denial of the existence of a material world (which is far from a correct view of it), was followed up by Hume, who, on similar reasoning, denied the existence of mind, and resolved the universe into a mere flow of ideas and impressions without any subject to be impressed, acknowledging, nevertheless, that he felt himself unable, practically, to acquiesce in his own unanswerable arguments. There was obviously some great mistake in a mode of reasoning that brought about a dead-lock of this description; and hence it has been the work of metaphysical philosophy since that time to endeavor to put the perception of the world on an admissible footing. Dr. Reid reclaimed against Berkeley and Hume by appealing to common-sense, or unreasoning instinct, as a sufficient foundation for our belief in the existence of a world apart from our own minds. Sir William Hamilton has expounded the same view with greater clearness and precision. He considers that our consciousness tells us at once that in the act of perceiving there is both a perceiving subject-self, or the mind — and an external reality, in relation with sense, as the object perceived. "Of the existence of both these things," he says, "I am convinced; because I am conscious of knowing each of them, not mediately in something else, as represented, but immediately in itself, as existing. Of their mutual dependence I am no less convinced, because each is apprehended equally and at once, in the same indivisible energy, the one not preceding or determining, the other not following or determined; and because each is apprehended out of and in direct contrast to the other" (Works, p. 747). Much as Hamilton has labored to elucidate this doctrine in all its bearings, it has not been universally accepted as satisfactory. Many believe that he has regarded as an ultimate fact of our constitution what admits of being still further resolved, and has mistaken an acquisition of the mature mind for a primitive or instinctive revelation. Professor Ferrier, in his Institutes of Metaphysics, has gone through the question with extraordinary minuteness and elaboration. His main position is the inseparability of the subject and the object in perception (a position also maintained by Hamilton in the above extract), which is not reconcilable with the common assumption as to the independent existence of matter. Indeed, he reduces the received dogma of the existence of matter per se to a self- contradiction, and builds up a system in strict conformity with the correlation, or necessary connection, of the mind perceiving with the object perceived. He thus approaches nearer to Berkeley than to Hamilton or to Reid. See Porter, Intellect; Ueberweg, Hist. of Philos.; South. Rev. Oct. 1873, art. 8, Westm. Rev. Jan. 1873, p. 119.