The most that can be said of any scientific hypothesis is, that whether true in the abstract or not, everything happens just as though it were true. When this test of universality is applied, when no known fact remains that is unexplained by it, the world is justified in assuming it to be true, and in deducing from it even the most momentous conclusions. If, on the contrary, there is one fact pertaining to the subject-matter under investigation which remains outside the domain of the hypothesis, or which is unexplained by it, it is indubitable evidence that the hypothesis is unsafe, untrue, and consequently worthless for all practical purposes of sound reasoning. Thus, Sir Isaac Newton, after having formulated his theorem, threw it aside as worthless, for a time, upon making the discovery that the moon, in its relations with the earth, apparently did not come within the terms of his hypothesis. His calculations were based upon the then accepted estimate of the length of a degree of latitude. This estimate having been corrected by the careful measurements of Picard, Newton revised his figures, and found that the supposed discrepancy did not exist. The last doubt in his mind having been thus set at rest, he gave to the world a theorem which rendered possible substantial progress in astronomical science. In the field of psychological investigation a satisfactory working hypothesis has never been formulated. That is to say, no theory has been advanced which embraces all psychological phenomena. Many theories have been advanced, it is true, to account for the various classes of phenomena which have been observed. Some of them are very plausible and satisfactory--to their authors--when applied to a particular class of facts, but utterly fail when confronted with another class.