The contrast between the naïve view of the criterion of reality and the one we have just been discussing may be brought out by considering how we should have to interpret from each standpoint the constant succession of facts in the history of science which have ceased to be facts. For illustration take the former fact that the earth is flat. It ceased to be a fact, says the theory we have been reviewing, because further thought-constructions of the real world convinced us that there is no reality which the idea "flat-world" represents. The idea "round-world" alone reproduces reality. It ceased to be a fact, says the naïve view, because it ceased to be a safe guide for action. Men found they could sail around the world. Correspondence in one case is pictorial, and its existence or non-existence can, as we have seen, never be ascertained. In the other, correspondence is response, adjustment, the co-meeting of specific conditions in further constituting of experience. In actual life, therefore, the criterion of reality which we use is a practical one. The test of reality does not consist in ascertaining the relationship between an idea and an _x_ which is not idea, but in ascertaining what experience can be taken for granted as a safe basis for securing other experiences. The evident advantage of the latter view, leaving aside for the moment the question of its adequacy in other respects, is that it avoids the fundamental skepticism at once suggested by the former. How can we ever be sure that the fact which we have discovered will stand the test of further thought-constructions? Perhaps it comes no nearer to reality than the discarded one. Obviously we never _can_ be sure that any particular content of thought represents reality so accurately and perfectly that it will never be subject to revision. If, however, the test of reality is the _adequacy_ of a given content of consciousness as a stimulus to action, as a mode of control, we have an applicable standard. A given content of consciousness is real--is a fact--so long as the act resulting from it is adequate in adaptation to other contents. It ceases to be real as soon as the act it stimulates proves to be inadequate.