The hypothesis is serviceable mainly in these particulars: (1) As a working basis. When one is confronted with a huge mass of facts it becomes necessary to start somewhere, and with as little waste of time and energy as possible. Almost anything is better than a haphazard floundering which reaches “nowhere.” So the investigator _hazards_ a tentative theory, which he at once proceeds to verify. If verification fails, then he may discard this first hypothesis for a better one. (2) As a guide to ultimate truth. Much might be said relative to the use of rejected hypotheses. By means of these, science has advanced step by step towards the full light of perfect knowledge. As has been remarked, no true scientist cares to overlook the opportunity for suggestive inspiration which some forsaken hypothesis may afford him. Just as the individual attains the best success by _using his failures as stepping stones_, so the true scientific discoverer _climbs up to the light on the stairway of discarded hypotheses_. By testing and rejecting the false hypotheses, the situation becomes more definite and the problem more accurately defined. “Kepler himself tried no less than nineteen different hypotheses before he hit upon the right one, and his ultimate success was doubtless in no slight degree due to his unsuccessful efforts.” (3) As a discoverer of immediate truth. Often, moreover, the hypothesis leads directly to positive verification. The supposition advanced may hit the truth squarely; and may be of such peculiar nature as to lead easily to clear and conclusive proof. (4) As affording a probable explanation of a problem which will not lend itself to an entirely satisfactory solution. The theory of evolution may illustrate this fourth use; while the history of the discovery of Neptune illustrates the third. =11. CHARACTERISTICS NEEDED BY SCIENTIFIC INVESTIGATORS.= The hypothesis is referred to “as the great instrument of science.” The greatest thinkers of time have possessed the courage and the conscience to step from the known to the unknown; to hazard a guess as to the meaning of what they saw, and then subject their guess to a rigorous test. This procedure involves three elements on the part of the investigator: (1) Power of accurate observation. (2) Constructive imagination. (3) A passion for truth. (1) An hypothesis formed without an accurate knowledge of facts is not only useless, but often it may work positive harm. To advance serviceable suppositions which are not grounded on fact, is as impossible, as it is to build a house without a foundation. The hypothesis is an image of the constructive imagination, but the pedestal of this image must rest on the ground of fact. The investigator who would be scientific must exercise scrupulous care in securing his facts through observation and experiment. The rules and errors involved in such a procedure have received sufficient attention.