_These Classes further distinguished._--Of the qualities thus derived, the primary are known immediately in themselves, the secondary only mediately in their effects on us, the secundo-primary both immediately in themselves, and mediately in their effects on us. The primary are qualities of body in relation to body simply, and to our organism as such; the secundo-primary are qualities of body in relation to our organism, not as body in general, but as body of a particular sort, viz.: propelling, resisting, cohesive; the secondary are qualities of body in relation to our organism as excitable and sentient. The primary may be roundly characterized as mathematical, the secundo-primary as mechanical, the secondary as physiological. _Reasons for retaining the twofold Division._--Such, in brief outline, are the principal points of Hamilton's classification. While following in the main the distinctions here indicated, I have preferred to retain the old division into primary and secondary, as at once more simple, and sufficiently accurate, merely dividing the secondary into two classes, the mechanical (secundo-primary of Hamilton), and physiological. We are thus enabled, not merely to retain a division and nomenclature which have antiquity and authority in their favor, and are well-nigh universally received, but we avoid the almost barbarous terminology of Sir William's classification--while, at the same time, we indicate with sufficient precision the important distinction between the so-called secundo-primary and secondary qualities. II. OF DIFFERENT THEORIES OF PERCEPTION. _Realists and Idealists._--There are two leading theories, quite distinct from each other, which have widely prevailed, and divided the thinking world, as to the philosophy of perception. The one maintains that in perception we have _direct cognizance_ of a real external world. This is the view taken in the preceding pages, and now generally held by psychologists in this country, and to some extent in Europe But for a long period, the prevalent, and in fact, until the time of Reid in Scotland, and Kant in Germany, the almost universally-received opinion was the reverse of this--that in perception, as in any and all other mental acts, the mind is conscious only of its own ideas, cognizant of itself and its own states only, incapable, in fact, of knowing any thing external to itself. Those who hold the former view are termed _Realists_, the latter _Idealists_.