_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.