[Sidenote: As to the criterion of truth--sense-delusions.] (4.) Of the possibility of a criterion of truth. An absolute criterion of truth must at once accredit itself, as well as other things. At a very early period in philosophy the senses were detected as being altogether untrustworthy. On numberless occasions, instead of accrediting, they discredit themselves. A stick, having a spark of fire at one end, gives rise to the appearance of a circle of light when it is turned round quickly. The rainbow seems to be an actually existing arch until the delusion is detected by our going to the place over which it seems to rest. Nor is it alone as respects things for which there is an exterior basis or foundation, such as the spark of fire in one of these cases, and the drops of water in the other. Each of our organs of sense can palm off delusions of the most purely fictitious kind. The eye may present apparitions as distinct as the realities among which they place themselves; the ear may annoy us with the continual repetition of a murmuring sound, or parts of a musical strain, or articulate voices, though we well know that it is all a delusion; and in like manner, in their proper way, in times of health, and especially in those of sickness, will the other senses of taste, and touch, and smell practise upon us their deceptions. This being the case, how shall we know that any information derived from such unfaithful sources is true? Pythagoras rendered a great service in telling us to remember that we have within ourselves a means of detecting fallacy and demonstrating truth. What is it that assures us of the unreality of the fiery circle, the rainbow, the spectre, the voices, the crawling of insects upon the skin? Is it not reason? To reason may we not then trust? [Sidenote: Uncertainties in philosophizing.]