convey that ‘sensitive knowledge of particular existence,’ which is the only possible knowledge of it. (Book IV. chap. iii. sec. 21.) As actual sensations and indices of reality, they do indeed differ in this or that ‘bare instance,’ but can convey no certainty that the real thing or ‘parcel of matter’ (Book III. chap. iii. sec. 18), which now causes the sensation of (and thus _is_) red, may not at another time cause the sensation of (and thus _be_) blue.’ [3] [1] Already in Book II. (chap. xxxi. sec. 12), the simple idea, as abstract, is spoken of as a nominal essence. [2] Cf. Book IV. chap. vi. sec. 5. ‘If we could certainly know (which is impossible) where a real essence, which we know not, is--_e.g._ in what parcels of matter the real essence of gold is; yet could we not be sure, that this or that quality could with truth be affirmed of gold; since it is impossible for us to know that this or that quality or idea has a necessary connexion with a real essence, of which we have no idea at all.’ Several passages, of course, can be adduced from Locke which are inconsistent with the statement in the text: _e.g._ Book IV. chap. iv. sec. 12. ‘To make knowledge real concerning substances, the ideas must be taken from the real existence of things. Whatever simple ideas have been found to coexist in any substance, these we may with confidence join together again, and so make abstract ideas of substances. For whatever have once had an union in nature, may be united again.’ In all such passages, however, as will appear below, the strict opposition between the real and the mental is lost sight of, the ‘nature’ or ‘substance,’ in which ideas ‘have a union,’ or are ‘found to coexist,’ being a system of relations which, according to Locke, it requires a mind to constitute, and thus itself a ‘nominal essence.’ [3] Cf. Book IV. chap. iii. sec. 29; Book IV. chap. vi. sec. 14; Book IV. chap. xi. sec. 11. How then is science of nature possible? 83. We thus come upon the crucial antithesis between relations of ideas and matters of fact, with the exclusion of general certainty as to the latter, which was to prove such a potent weapon of scepticism in the hands of Hume. Of its incompatibility with recognized science we can have no stronger sign than the fact that, after more than a century has elapsed since Locke’s premisses were pushed to their legitimate conclusion, the received system of logic among us is one which, while professing to accept Locke’s doctrine of essence, and with it the antithesis in question, throughout assumes the possibility of general propositions as to matters of fact, and seeks in their methodical discovery and proof that science of nature which Locke already ‘suspected’ to be impossible. (Book IV. chap. xii. sec. 10.) No ‘uniformities of phenomena’ can be known.