Those metaphysicians who fared farthest from the philosopher of Cusa evolved the doctrine that, in themselves, things have no existence at all, and that we can think of them only in terms of our impressions of them. The color green, for example, may be nothing but a delusion, for all we can possibly know of it is that, under certain conditions, our optic nerves experience a sensation of greenness. Whether this sensation of greenness is a mere figment of our imagination or the reflection of an actual physical state, is something that we cannot tell. It is impossible, in a word, to determine whether there are actual things around us, which produce real impressions upon us, or whether our idea of these things is the mere result of subjective impressions or conditions. We know that a blow on the eyes may cause us to see a flash of light which does not exist and that a nervous person may feel the touch of hands and hear noises which are purely imaginary. May it not be possible, also, that all other sensations have their rise within us instead of without, and that in saying that objects give us impressions we have been confusing cause and effect? Such is the argument of those metaphysicians who doubt, not only the accuracy of human knowledge, but also the very capacity of human beings to acquire knowledge. It is apparent, on brief reflection, that this attitude, while theoretically admissible, is entirely impracticable, and that, as a matter of fact, it gives us no more substantial basis for intelligent speculation than the old device of referring all questions to revelation. To say that nothing exists save in the imagination of living beings is to say that this imagination itself does not exist. This, of course, is an absurdity, because every man is absolutely certain that he himself is a real thing and that his mind is a real thing, too, and capable of thought. In place of such cob-web spinning, modern philosophers--driven to it, it may be said, in parenthesis, by the scientists--have gone back to the doctrine that, inasmuch as we can know nothing of anything save through the impressions it makes upon us, these impressions must be accepted provisionally as accurate, so long as they are evidently normal and harmonize one with the other.