involving that contradiction which it must involve if the antithesis between reality and ideas in the mind is absolutely adhered to. Nay, inasmuch as certain ideas of primary qualities, _e.g._ those of solidity and of the receiving or communicating motion upon impulse, are necessarily connected, it is supposed actually to exist (Book IV. chap iii. sec. 14). It is only because, as a matter of fact, our knowledge of the relation between secondary qualities and primary is so limited that it cannot be carried further. That they are related as effects and causes, it would seem, we know; and that the ‘causes work steadily, and effects constantly flow from them,’ we know also; but ‘their connexions and dependencies are not discoverable in our ideas’ (Book IV. chap. iii. sec. 29). That, if discoverable in our ideas, just because there discovered, the connexion would not be a real co-existence, Locke never expressly says. He does not so clearly articulate the antithesis between relations of ideas and matters of fact. If he had done so, he must also have excluded from real existence those abstract ideas of body which constitute the scanty knowledge of it that according to him we do possess (Book IV. chap. iii. sec. 24). He is more disposed to sigh for discoveries that would make physics capable of the same general certainty as mathematics, than to purge the former of those mathematical propositions--really true only because having no reference to reality--which to him formed the only scientific element in them. What knowledge it can afford, according to Locke.