(22) We see by a passage from the fourth part of the Discourse on Method, by Descartes, cited in note xv., that besides the principle, I think, therefore I am, he admitted the principle of the legitimacy of evidence; for, asking what is necessary in order that a proposition may be true and certain, he says, that having remarked that if he was certain of the truth of this proposition (I think, therefore I am), he was so only because he saw it to be so; he believed that he could take it to be a general rule that things known clearly and distinctly are all true. This shows two principles, closely connected, although very unlike, to enter into Descartes' system. The first is the fact of consciousness of thought; the second is the general rule of the legitimacy of the criterion of evidence.
It is also to be remarked that there is here some confusion of ideas, which we have already pointed out. It is not exact to say that the principle, I think, therefore I am, is evident: the evidence relates to the consequence; but there is, properly speaking, no evidence of the act of thought, excepting consciousness. Evidence is a criterion, but not the only one.
ON CHAPTER XXIII.
(23) What we have said of the second proposition of this chapter is independent of the mode in which the soul and body exercise their mutual influence. These questions belong elsewhere. This influence is, in every system, a fact attested by experience; and this is all that is needed for what we propose to establish here.
ON CHAPTER XXIV.
(24) For the better understanding of what we have said in this chapter on evidence, it will be well to consider what will be advanced in chapters XXVI. to XXXI., inclusive.
ON CHAPTER XXV.
(25) What has been said in this chapter shows the truth of what we said in the twenty-fourth chapter, upon the connection of the different criteria, and the necessity of not confining one's self to an exclusive philosophy. Consciousness serves as the basis of the other criteria, as an indispensable fact; but if we deny the others, we also deny consciousness.
ON CHAPTERS XXVI., XXVII., AND XXVIII.