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:--