matter of course) that he has a secure foundation for the validity of knowledge in general: for a perfect being cannot deceive.
Descartes bases the knowledge of reality on the idea of God, just as Kepler had explained the conformity of nature to mathematical principles on theological grounds. But, in that case, God is merely an explanation of the sublime uniformity of natural phenomena, rather than a specifically religious concept. Thus, e.g. in the sixth meditation, he says, “By nature in general (natura generaliter spectata) I simply mean God himself or the order and disposition instituted (coordinatio) by Him in created things.” Everything which is to be accepted as true must fit into this great system. The criterion by which we are able to distinguish between dream and wakeful consciousness consists in the fact that the various experiences of wakeful life can be coordinated with our total experiences and recollections without a break in the system.—Descartes had not observed that this criterion was already contained in the causal principle, so that he might have spared himself the indirect route through the idea of God. The establishment of this criterion furnished the basis of a new conception of truth, according to which truth consists of the internal relation of perceptions and ideas, instead of their harmony with something unperceived.
Descartes is fully aware that the idea of God, which he makes the foundation of all science, is not the popular one. He says that when God is conceived as a finite being, receiving honor from men, it is not strange that His existence should be denied. God is however the absolute Substance, i.e. a being, which exists through itself (per se), requires no other being, in order to exist. It is true, Descartes likewise employs the concept of substance in
