Now, in considering this question we must first remember to what an extraordinarily high level of adaptive ideation the purely receptual life of brutes is able to carry them. If we contrast the ideation of my cebus, which honestly investigated the mechanical principle of a screw, and then applied his specially acquired knowledge to screws in general—if we contrast this ideation with that of palæolithic man, who for untold thousands of years made no advance upon the chipping of flints, we cannot say that, when gauged by the practical test of efficiency or adaptation, the one appears to be very much in advance of the other. Or, if we remember that these same men never hit upon the simple expedient of attaching a chipped flint to a handle, so as to make a hatchet out of a chisel,[129] it cannot be said that in the matter of mechanical discovery early conceptual life displayed any great advance upon the high receptual life of my cebus. Nevertheless, I have allowed—nay insisted—that no matter how elaborate the structure of receptual knowledge may be, or how wonderful the adaptive action it may prompt, a “practical inference” or “receptual judgment” is always separated from a conceptual inference or true judgment by the immense distinction that it is not itself an object of knowledge. No doubt it is a marvellous fact that by means of receptual knowledge alone a monkey should be able to divine the mechanical principle of _a screw_, and afterwards apply his discovery to all cases of _screws_. But even here there is nothing to show that the monkey ever _thought_ about the principle _as_ a principle; indeed, we may rest well assured that he cannot possibly have done so, seeing that he was not in possession of the intellectual instruments—and, therefore, of the _antecedent conditions_—requisite for the purpose. All that the monkey did was to perceive receptually certain analogies: but he did not _conceive_ them, or constitute them objects of thought _as_ analogies. He was, therefore, unable to _predicate_ the discovery he had made, or to set before his own mind as knowledge the knowledge which he had gained.