If we ask the philosophically unsophisticated individual why he believes that his house still exists when he is away from it and has no immediate evidence of the fact, he will tell you it is because he has found that he can go back to it time and again and see it and walk into it. It never fails him when he acts upon the assumption that it is there. He would never tell you that he believed in its existence when he was not experiencing it because his mental picture of his house stood for and represented accurately an object in the real world which was nevertheless of a different order of existence from his mental picture. When you ask the physicist why he believes that the laws of motion are true, he will tell you that it is because he finds that bodies always do behave according to them. He can predict just what a body will do under given circumstances. He is never disappointed however long he takes it for granted that the laws of motion are true and that bodies behave according to them. The only thing that could make him question their truth would be to find some body which did not prove to behave in accordance with them. The criterion is the same in both cases. It is the practical criterion of what as a matter of fact will work. That which can safely be taken for granted as a basis for further action is regarded as real and true. It remains real so long, and only so long, as it continues to fulfil this condition. As soon as it ceases to do so, it ceases to be regarded as real. When a man finds that he can no longer obtain the accustomed experience of seeing and entering his house, he ceases to regard it as real. It has burned down, or been pulled down. When a physicist finds that a body does not, as a matter of fact, behave as a given law leads him to expect it would behave, he ceases to regard the law as _true_.
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.