Indeed, it is not too much to say that we have no really profound knowledge upon any subject—no knowledge on the strength of which we are ready to act at all moments unhesitatingly without either preparation or after-thought—till we have left off feeling conscious of the possession of such knowledge, and of the grounds on which it rests. A lesson thoroughly learned must be like the air which feels so light, though pressing so heavily against us, because every pore of our skin is saturated, so to speak, with it on all sides equally. This perfection of knowledge sometimes extends to positive disbelief in the thing known, so that the most thorough knower shall believe himself altogether ignorant. No thief, for example, is such an utter thief—so _good_ a thief—as the kleptomaniac. Until he has become a kleptomaniac, and can steal a horse as it were by a reflex action, he is still but half a thief, with many unthievish notions still clinging to him. Yet the kleptomaniac is probably unaware that he can steal at all, much less that he can steal so well. He would be shocked if he were to know the truth. So again, no man is a great hypocrite until he has left off knowing that he is a hypocrite. The great hypocrites of the world are almost invariably under the impression that they are among the very few really honest people to be found and, as we must all have observed, it is rare to find any one strongly under this impression without ourselves having good reason to differ from him. Our own existence is another case in point. When we have once become articulately conscious of existing, it is an easy matter to begin doubting whether we exist at all. As long as man was too unreflecting a creature to articulate in words his consciousness of his own existence, he knew very well that he existed, but he did not know that he knew it. With introspection, and the perception recognised, for better or worse, that he was a fact, came also the perception that he had no solid ground for believing that he was a fact at all. That nice, sensible, unintrospective people who were too busy trying to exist pleasantly to trouble their heads as to whether they existed or no—that this best part of mankind should have gratefully caught at such a straw as “_cogito ergo sum_,” is intelligible enough. They felt the futility of the whole question, and were thankful to one who seemed to clench the matter with a cant catchword, especially with a catchword in a foreign language; but how one, who was so far gone as to recognise that he could not prove his own existence, should be able to comfort himself with such a begging of the question, would seem unintelligible except upon the ground of sheer exhaustion.