Again, and this is a point of the first importance, experience never gives us the “primary quality” by itself. What we get in actual experience is always the conjunction of primary and secondary qualities in a concrete perception. Thus we never perceive extension apart from some special visual or tactual filling of the “secondary” kind. The extended has always some quality of colour, or texture, or resistance. An extension which is totally devoid of colour, tactual quality, and everything which belongs to the so-called sensible, non-mathematical, or “secondary” properties of body, is an unreal abstraction, got by leaving out an aspect which in actual experience appears inseparable from it, and therefore presumably illegitimate. Illegitimate, that is, when offered as an account of the fundamental reality of body, however useful for the special purposes of natural science. Thus the attempt to take the so-called primary qualities as the unitary “substance” which has or “possesses” the secondary qualities, and to dispose of these latter as “subjective,” leads to no satisfactory result. The former, too, must be merely qualities possessed by a more ultimate substance. § 5. Hence it constantly happens that the same writers who treat substance as identical with the primary qualities of things, alternate this view with another according to which substance is an unknowable unit of which we can say no more than that it, whatever it may be, is what is presupposed in all propositions about the behaviour of things as the “unknown substratum” of their various qualities. According to this view, the many qualities of the thing in some inexplicable manner “flow” either from the nature of its own unknown substratum or substance, or from the relations in which this substratum stands with that of other things.[79] Our knowledge is then held to be confined to these consequences of the unknown ultimate character of real things; we are ignorant, it is said, of the substance both of physical and of mental existence, we know only its attributes or manifestations. Or it is otherwise phrased thus: we do not know what things really are, we know only their effects on one another and on our own senses. This is, for instance, the view represented by those portions of Locke’s Essay in which emphasis is laid upon our inability in the last resort to know the true substance of things.