The Moderns.—Descartes and Locke merely adopted these distinctions as they found them, without essential modification. So also did Reid and Stewart, although both included among the primary qualities some which are properly secondary, as roughness, smoothness, hardness, softness. Indeed Stewart restricted the primary qualities to those and such as those just named.
Hamilton.—No writer has so fully elaborated this matter as Sir William Hamilton, to whom we are indebted mainly for the historical facts now stated, and whose dissertations are and must ever remain an invaluable thesaurus on the philosophy of perception. So complete and elaborate is his classification of the qualities of matter, that I shall be pardoned for giving a synopsis of its principal points in this connection.
Hamilton's Scheme—General Divisions.—He divides the qualities of bodies into three classes, which he calls primary, secundo-primary, and secondary. The primary are thought as essential to the very notion of matter, and may be deduced à priori, the bare notion of matter being given; while the secundo-primary and the secondary, being accidental and contingent, must be deduced à posteriori, learned by experience. His deduction of the primary qualities is as follows:
Primary Qualities.—We can conceive of body only as, I. Occupying space; II. Contained in space. Space is a necessary form of thought, but we are not obliged to conceive of space as occupied, that is, to conceive of matter. When conceived it must be under the conditions now named.
I. The property of occupying space is Simple Solidity, which implies, a. Trinal extension, or length, breadth, and thickness; b. Impenetrability, or the property of not being reduced to non-extension. Trinal extension involves, 1. Number, or Divisibility; 2. Size, including Density; 3. Shape.
II. The attribute of being contained in space, affords the notion, 1. Of Mobility; 2. Of Position.
The essential and necessary constituents then of our notion of matter are, 1. Extension (comprising under it, 2. Divisibility; 3. Size; 4. Density; 5. Figure); 6. Ultimate Incompressibility; 7. Mobility; 8. Situation. These are the primary qualities, products, in a sort, of the understanding, developing themselves with rigid necessity out of the given notion of substance occupying space.
Secundo-Primary Qualities.—The secundo-primary are contingent modifications of the primary, all have relation to space, and motion in space, all are contained under the category of resistance, or pressure, all are learned or included as results of experience, all have both an objective and subjective phase, being at once qualities of matter, and also affections of our senses.
Considered as to the sources of resistance, there is, I. That of Co-attraction, under the forms of a, Gravity, b, Cohesion; II. That of Repulsion; III. Inertia; all which are capable of minute subdivision. Thus from cohesion follow the hard and soft, firm and fluid, tough and brittle, rigid and flexible, rough and smooth, etc., etc. From repulsion are derived compressible and incompressible, resilient and irresilient.
Secondary Qualities.—The secondary qualities are, as apprehended by us, not properly attributes of body at all, but only affections of our nervous organism. They belong to bodies only so far as these are furnished with the power of exciting our nervous organism to the specific action thus designated. To this class belong color, sound, flavor, savor, tactile sensation, feeling of heat, electricity, etc. Such also are titillation, sneezing, shuddering, and the various sensations, pleasurable or painful, resulting from the action of external stimuli.