The answer of common sense is that we are immediately conscious, in perception, of an _ego_ and a _non-ego_ known together, and known in contrast to each other; we are conscious of a perceiving subject, and of an external reality, as the object perceived.[244] To state this doctrine of natural realism still more explicitly we add, that we are conscious of the immediate perception of certain essential attributes of matter objectively existing. Of these primary qualities, which are immediately perceived as real and objectively existing, we mention _extension_ in space and _resistance_ to muscular effort, with which is indissolubly associated the idea of _externality_. It is true that extension and resistance are only qualities, but it is equally true that they are qualities of something, and of something which is external to ourselves. Let any one attempt to conceive of extension without something which is extended, or of resistance apart from something which offers resistance, and he will be convinced that we can never know qualities without knowing substance, just as we can not know substance without knowing qualities. This, indeed, is admitted by Mr. Mill.[245] And if this be admitted, it must certainly be absurd to speak of substance as something "unknown." Substance is known just as much as quality is known, no less and no more. [Footnote 244: Hamilton, "Lectures," vol. 1. p. 288.] [Footnote 245: "Logic," bk. i. ch. iii. § 6.] We remark, in conclusion, that if the testimony of consciousness is not accepted in all its integrity, we are necessarily involved in the Nihilism of Hume and Fichte; the phenomena of mind and matter are, on analysis, resolved into an absolute nothingness--"a play of phantasms in a void."[246] (ii.) We turn, secondly, to the _Materialistic School_ as represented by Aug. Comte. The doctrine of this school is that all knowledge is limited to _material_ phenomena--that is, to appearances _perceptible to sense_. We do not know the essence of any object, nor the real mode of procedure of any event, but simply its relations to other events, as similar or dissimilar, co-existent or successive. These relations are constant; under the same conditions, they are always the same. The constant resemblances which link phenomena together, and the constant sequences which unite them, as antecedent and consequent, are termed _laws_. The laws of phenomena are all we know respecting them. Their essential nature and their ultimate causes, _efficient_ or _final_, are unknown and inscrutable to us.[247] [Footnote 246: Masson, "Recent British Philos.," p. 62.] [Footnote 247: See art. "Positive Philos. of A. Comte," _Westminster Review_, April, 1865, p. 162, Am. ed.] It is not our intention to review the system of philosophy propounded by Aug. Comte; we are now chiefly concerned with his denial of all causation.