On this subject of _perception_ it is well known that Reid and Stewart, refusing to be drawn into any hypothesis or unsatisfactory analysis, contented themselves with stating, in the preciser language of the schools, the _fact_ as it appears to the plain unsophisticated observer. Reid's explanations are unfortunately mingled up with his controversy against the old hypothesis of _ideas_ or _images_ of things perceived in the mind--an hypothesis combated by him with unnecessary vehemence--but this detracts little from their substantive correctness or utility. This strange notion of images emanating from the external object, entering the mind, and being there perceived, was, after all, in its origin, rather a physical than a metaphysical hypothesis. The ancient speculator upon the causes of things felt, as we feel at this moment, the necessity for some medium of communication between the eye and the distant object, and not having detected this medium in the light which traverses or fills the space between them, he had recourse to this clumsy invention of _images_ or _species_ raying out from the surfaces of things. At the time when Reid wrote, this hypothesis, in its crude form, cannot be said to have existed; but it had left its traces in the philosophical language of the period, and there was certainly a vague notion prevalent that the idea of an object was a _tertium quid_, a something that was neither the mind nor the object.
We will quote the statement which Dugald Stewart makes of Reid's doctrine of perception. As he himself adopts the statement, it will embrace at once the opinion of both these philosophers:--
"To what, may it be asked, does this statement (of Reid's) amount? Merely to this, that the mind is so formed that certain impressions produced on our organs of sense by external objects, are followed by correspondent sensations, and that these sensations (which have no more resemblance to the qualities of matter, than the words of a language have to the things they denote) are followed by a perception of the existence and qualities of the bodies by which the impressions are made; that all the steps of this progress are equally incomprehensible; and that for any thing we can prove to the contrary, the connexion between the sensation and the perception, as well as that between the impression and the sensation, may be both arbitrary; that it is therefore by no means impossible that our sensations may be merely the occasions on which the correspondent perceptions are excited; and that at any rate the consideration of these sensations, which are attributes of mind, can throw no light on the manner in which we acquire our knowledge of the existence and qualities of body. From this view of the subject, it follows that it is the external objects themselves, and not any species or images of these objects (_or, we may add, any mere agglomeration of present and remembered sensations_) that the mind perceives; and that although, by the constitution of our nature, certain sensations are rendered the constant antecedents of our perceptions, yet it is just as difficult to explain how our perceptions are obtained by their means, as it would be upon the supposition that the mind were all at once inspired with them, without any concomitant sensations whatever."--(_Elements of the Philosophy of the Human Mind_, Vol. i. p. 92.)