88. It remains to exhibit briefly the disguises under which these inherent difficulties of his theory of essence appear in Locke. Throughout, instead of treating ‘essence’ altogether as a fiction of the mind--as it must be if feelings in simplicity and singleness are alone the real--he treats indeed as a merely ‘nominal essence’ every possible combination of ideas of which we can speak, but still supposes another essence which is ‘real.’ But a real essence of what? Clearly, according to his statements, of the same ‘thing’ of which the combination of ideas in the mind is the nominal essence. Indeed, there is no meaning in the antithesis unless the ‘something,’ of which the latter essence is so nominally, is that of which it is not so really. So says Locke, ‘the nominal essence of gold is that complex idea the word gold stands for; let it be, for instance, a body yellow, of a certain weight, malleable, fusible, and fixed. But the real essence is the constitution of the insensible parts _of that body_, on which those qualities and all the other properties of gold depend.’ (Book III. chap. vi. sec. 2.) Here the notion clearly is that of one and the same thing, of which we can only say that it is a ‘body,’ a certain complex of ideas--yellowness, fusibility, &c.--is the nominal, a certain constitution of insensible parts the real, essence. It is on the real essence, moreover, that the ideas which constitute the nominal depend. Yet while they are known, the real essence (as appears from the context) is wholly unknown. In this case, it would seem, the cause is not known from its effects. According to one, it is a collection of ideas as qualities of a thing: