[253] Vol. ii., p. 305. See too the passage, quoted above, in his posthumous dialogue. 105. Thus the logic of Descartes, using that word for principles that guide our reasoning, was an instrument of defence both against the captiousness of ordinary scepticism, that of the Pyrrhonic school, and against the disputatious dogmatism of those who professed to serve under the banner of Aristotle. He who reposes on his own consciousness, or who recurs to first principles of intuitive knowledge, though he cannot be said to silence his adversary, should have the good sense to be silent himself, which puts equally an end to debate. But so far as we are concerned with the investigation of truth, the Cartesian appeal to our own consciousness, of which Stewart was very fond, just as it is in principle, _may_ end in an assumption of our own prejudices as the standard of belief. Nothing can be truly self-evident, but that which a clear, an honest, and an experienced understanding in another man acknowledges to be so. |Treatise on art of logic.| 106. Descartes has left a treatise highly valuable, but not very much known, on the art of logic, or rules for the conduct of the understanding.[254] Once only, in a letter, he has alluded to the name of Bacon.[255] There are perhaps a few passages in this short tract that remind us of the Novum Organum. But I do not know that the coincidence is such as to warrant a suspicion that he was indebted to it; we may reckon it rather a parallel, than a derivative logic; written in the same spirit of cautious, inductive procedure, less brilliant and original in its inventions, but of more general application than the Novum Organum, which is with some difficulty extended beyond the province of natural philosophy. Descartes is as averse as Bacon to syllogistic forms. “Truth,” he says, “often escapes from these fetters, in which those who employ them remain entangled. This is less frequently the case with those who make no use of logic, experience showing that the most subtle of sophisms cheat none but sophists themselves, not those who trust to their natural reason. And to convince ourselves how little this syllogistic art serves towards the discovery of truth, we may remark that the logicians can form no syllogism with a true conclusion, unless they are already acquainted with the truth that the syllogism develops. Hence it follows that the vulgar logic is wholly useless to him who would discover truth for himself, though it may assist in explaining to others the truth he already knows, and that it would be better to transfer it as a science from philosophy to rhetoric.”[256]