_Difference of Qualities._--The qualities of bodies as known to us through sensation and perception are many and various. On examination, a difference strikes us as existing among these qualities, which admits of being made the basis of classification. Some of them are qualities which strike us at once as essential to the very existence of matter, at least in our notion of it, so that we cannot in thought divest it of these qualities, and still retain our conception of matter. Others are not of this nature. Extension, divisibility, size, figure, situation, and some others, are of the former class. If matter exists at all, it must, according to our own conceptions, possess these qualities. We cannot think them away from it, and leave matter still existing. But we can conceive of matter as destitute of color, flavor, savor, heat, cold, weight, sound, hardness, etc. These are contingent and accidental properties not necessary to its existence. _How named and distinguished._--Philosophers have called the former class _primary_, the latter _secondary_ qualities. The former are known _à priori_, the latter by experience. The former are known as qualities, in themselves, the latter only through the affections of our senses. The primary qualities then have these characteristics: 1. They are essential to the very existence of matter, at least in our conception. 2. They are to be known _à priori_. 3. They are known as such, or in themselves. The secondary, on the contrary, are: 1. Accidental, not essential to the notion of matter. 2. To be known only by experience. 3. To be learned only through the affection of the senses. _Further Division of secondary Qualities._--A further division, however, is capable of being made. The secondary qualities, as now defined, comprise, in reality, two classes. There are some, which, while known to us only through the senses, have still an existence as qualities of external objects, independent of our senses. As such they are objects of direct perception. Others, again, are known, not as qualities of bodies, but only as affections of sense, not as objective, but only as subjective, not as perceptions, but only as sensations. Thus I distinguish the smell, the taste, and the color of an orange. What I distinguish, however, is after all only certain sensations, certain affections of my own organism. What may be the peculiar properties or qualities in the object itself which are the exciting cause of these sensations in me, I know not. My perception does not extend to them at all. It is quite otherwise with the qualities of weight, hardness, compressibility, fluidity, elasticity, and others of that class. They are objects of perception, and not of sensation merely.