of the existence of things outside him exactly like his ideas as something which “I thought I perceived very clearly, though in reality I did not perceive it all”. In this case, however, the Empirical criterion offers a guarantee against error by the rigorous separation of observation from inference. This guarantee I will now proceed to examine.
I may begin by remarking a curious interchange of rôles between Rationalism and Empiricism as regards the evidence claimed for their respective criteria. While the Rationalist’s criterion is partly supported, as we have seen, on an appeal to experience, the validity of the Empirical criterion appears to be treated as self-evident. At least this seems to be implied in Mill’s language before referred to; where, after pointing out various possible sources of error in the affirmation that “I saw my brother this morning,” he says that if any of these possibilities had been realised, “the affirmation that I saw my brother would have been erroneous: but whatever was matter of direct perception, namely the visual sensations, would have been real”. For his argument requires us to understand the last sentence as meaning not merely that there would have been sensations for me to perceive, but that my perception of them would certainly have been free from error: and as no empirical proof is offered of this last proposition, it seems to have been regarded as not requiring proof. But—even if we assume, to limit the discussion, that a man cannot, strictly speaking, observe anything except his own states of consciousness—it still seems paradoxical to affirm that the elimination of all inference from such observation would leave a residuum of certainly true cognition: considering the numerous philosophical disputes that have arisen from the conflicting views taken by different thinkers of psychical experiences supposed to be similar. Take (e.g.) the controversy since Hume about the impossibility of finding a self in the stream of psychical experience, or that as to the consciousness of free will, or the disinterestedness of moral choice, or the feeling-tone of desire; surely in view of these and other controversies it would be extraordinarily rash to claim freedom from error for our cognitions of psychical fact, let them be never so rigorously purged of inference.
The truth seems to be that the indubitable certainty of the judgment “I am conscious” has been rather hastily extended by Empiricists to judgments affirming that my present consciousness is such and such. But these latter judgments necessarily involve an implicit comparison and classification of the present consciousness with elements
