"4. To make such exact calculations, and such circumspections as to be confident that nothing essential has been omitted." "Consciousness being the basis of all certitude, everything of which you are clearly and distinctly conscious must be true: everything which you clearly and distinctly conceive, exists, if the idea involve existence." It should be remarked that consciousness being a state of condition of the mind, is by no means an infallible guide? Men may fancy they have clear ideas, when their consciousness, if carefully examined, would prove to have been treacherous. Descartes argued for three classes of ideas--acquired, compounded, and innate. It is in his assumption of innate ideas that you have one of the radical weaknesses of his system. Sir William Hamilton points out that the use of the word idea by Descartes, to express the object of memory, imagination, and sense, was quite a new usage, only one other writer, David Buchanan, having previously used the word idea with this signification. Descartes did not write for the mass, and his philosophy would have been limited to a much narrower circle had its spread rested on his own efforts. But the age was one for new thought, and the contemporaries and successors of Descartes carried the Cartesian logic to extremes he had perhaps avoided, and they taught the new philosophy to the world in a fearless spirit, with a boldness for which Descartes could have given them no example. Descartes, who in early life had travelled much more than was then the custom, had probably made the personal acquaintance of most of the leading thinkers of Europe then living; it would be otherwise difficult to account for the very ready reception given by them to his first work. Fortunately for Descartes, he was born with a fair fortune, and escaped such difficulties as poorer philosophers must needs submit to. There is perhaps a _per contra_ side. It is more than possible that if the needs of life had compelled him, Descartes' scientific predilections might have resulted in more immediate advantage to society. His philosophy is often pedantic to weariness, and his scientific theories are often sterile. The fear of poverty might have quickened some of his speculations into a more practical utterance. Buckle reminds us that Descartes "was the first who successfully applied algebra to geometry; that he pointed out the important law of the sines; that in an age in which optical instruments were extremely imperfect, he discovered the changes to which light is subjected in the eye by the crystalline lens; that he directed attention to the consequences resulting from the weight of the atmosphere, and that he detected the causes of the rainbow." "Descartes," says Saintes, "throwing off the swaddling clothes of scholasticism, resolved to owe to himself alone the acquisition of the truth which he so earnestly desired to possess. For what else is the methodical doubt which he established as the starting