123. To a reader, fresh from our popular treatises on Logic, such language would probably at first present no difficulty. He would merely lament that Locke, as a successor of Bacon, was not better acquainted with the ‘Inductive methods,’ and thus did not understand how an observation of co-existence in the bare instance, if the instance be of the right sort, may warrant a universal affirmation. Or he may take the other side, and regard Locke’s restriction upon general certainty as conveying, not any doubt as to the validity of the inference from an observed case to all cases where the conditions are ascertainably the same, but a true sense of the difficulty of ascertaining in any other case that the conditions are the same. On looking closer, however, he will see that, so far from Locke’s doctrine legitimately allowing of such an adaptation to the exigencies of science, it is inconsistent with itself in admitting the reality of most of the conditions in the case supposed to be observed, and thus in allowing the real truth even of the singular proposition. This purports to state, according to Locke’s terminology, that certain ‘ideas’ do now or did once co-exist in a body. But the ideas, thus stated to co-exist, according to Locke’s doctrine that real existence is only testified to by actual present sensation, differ from each other as that which _really_ exists from that which does not. In the particular experiment of gold being solved in aqua regia, from the complex idea of solubility an indefinite deduction would have to be made for qualification by ideas retained in the understanding before we could reach the present sensation; and not only so, but the group of ideas indicated by ‘gold,’ to whose co-existence with solubility the experiment is said to testify, as Locke himself says, form merely a nominal essence, while the body to which we ascribe this essence is something which we ‘accustom ourselves to suppose,’ not any ‘parcel of matter’ having a real existence in nature. [1] In asserting the co-existence of the ideas forming such a nominal essence with the actual sensation supposed to be given in the experiment, we change the meaning of ‘existence,’ between the beginning and end of the assertion, from that according to which all ideas exist to that according to which existence has no ‘connexion with any other of our ideas but those of ourselves and God,’ but is testified to by present sensation. [2] This paralogism escapes Locke just as his equivocal use of the term ‘idea’ escapes him. The distinction, fixed in Hume’s terminology as that between impression and idea, forces itself upon him, as we have seen, in the Fourth book of the Essay, where the whole doctrine of real existence turns upon it, but alongside of it survives the notion that ideas, though ‘in the mind’ and forming a nominal essence, are yet, if rightly taken from things, ectypes of reality. Thus he does not see that the co-existence of ideas, to which the particular