He begins his great work by examining into the permanent causes of error, as these were likely to be operative even after the reformation of science. For this reason he calls them idols, or false appearances (from the Greek, _eidolon_), and he divides them into four classes: the idols of the tribe, or the causes of error due to the general defects of the human mind; the idols of the den, which spring from weaknesses peculiar to the character of the individual student; the idols of the forum, which arise out of the intercourse of society and the power that words sometimes have of governing thought; and, finally, the idols of the theatre, which men of great learning pursue when they follow the systems of famous but mistaken thinkers. After this preliminary discussion, Bacon goes on to describe the methods of inductive science. The first step consists in preparing a history of the phenomena to be explained in all their modifications and varieties. This history must include not merely such facts as spontaneously offer themselves, but all experiments instituted for the sake of discovery. It must be composed with great care; the facts should be accurately related and distinctly arranged, and their authenticity diligently examined; those that rest on doubtful evidence should not be rejected, but noted as uncertain, with the grounds of the judgment so formed. This last part of the method, says Bacon, is very necessary, for facts often appear incredible only because we are ill-informed, and they cease to seem marvellous when our knowledge is further extended. When this record of facts, this "natural history," is completed, an attempt may then be made to discover, by a comparison of the various facts, the cause of the phenomena. Here it is of the utmost importance to bear in mind that all facts have not the same value. There are, as Bacon points out, twenty-seven species of facts, and he concludes that in any science where facts cannot be tested by experiment there can be no conclusive evidence. Thus it will be seen that Bacon's method was a system of specific rules. He did not merely tell men to make observations and experiments; he taught them how observations and experiments ought to be made. As Bacon was the father of modern science, so Réne Descartes was the father of modern philosophy. Born in 1596, and perplexed by the movement of scepticism produced by the Renaissance, the French thinker endeavoured to find some ground of certainty in the fact that he at least knew of his own existence. Hence his famous saying: _Cogito, ergo sum_--"I think, therefore I exist." Consciousness, said he, is the basis of all knowledge. The process then is simple: examine your consciousness, and its clear replies will be science. Hence the vital portion of his system lies in this axiom: "All clear ideas are true."