121. It may find some excuse for itself in the hesitation with which Locke pronounces the impossibility of real generality in the knowledge of nature--an hesitation which necessarily results from the ambiguities, already noticed, in his doctrine of real and nominal essence. So far as the opposition between the nominal and real essences of substances is maintained in its absoluteness, as that between every possible collection of ideas on the one side, and something wholly apart from thought on the other, this impossibility follows of necessity. But so far as the notion is admitted of the nominal essence being in some way, however inadequately, representative of the real, there is an opening, however indefinite, for general propositions concerning the latter. On the one hand we have the express statement that ‘universal propositions, of whose truth and falsehood we can have certain knowledge, concern not existence’ (Book IV. chap. ix. sec. 1). They are founded only on the ‘relations and habitudes of abstract ideas’ (Book IV. chap xii. sec. 7); and since it is the proper operation of the mind in abstraction to consider an idea under no other existence but what it has in the understanding, they represent no knowledge of _real_ existence at all (Book IV. chap. ix. sec. 1). Here Locke is consistently following his doctrine that the ‘particularity in time,’ of which abstraction is made when we consider ideas as in the understanding, is what specially distinguishes the real; which thus can only be represented by ‘actually present sensation.’ It properly results from this doctrine that the proposition representing particular experiment and observation is only true of real existence so long as the sensation, in which the experiment consists, continues present. Not only is the possibility excluded of such experiment yielding a certainty which shall be general as well as real, but the particular proposition itself can only be _really_ true so far as the qualities, whose co-existence it asserts, are present sensations. The former of these limitations to real truth we find Locke generally recognising, and consequently suspecting a science of nature to be impossible; but the latter, which would be fatal to the supposition of there being a real nature at all, even when he carries furthest the reduction of reality to present feeling, he virtually ignores. On the other hand, there keeps appearing the notion that, inasmuch as the combination of ideas which make up the nominal essence of a substance is taken from a combination in nature or reality, whenever the connexion between any of these is necessary, it warrants a proposition _universally_ true in virtue of the necessary connexion between the ideas, and _really_ true in virtue of the ideas being taken from reality. According to this notion, though ‘the certainty of universal propositions concerning substances is very narrow and scanty,’ it is yet possible (Book IV. chap. vi. sec. 13). It is not recognised as