HYPOTHESIS (Gk. i.760earc , supposition, from i.7,-oriOepat, hypotithenai, to place under, from 076, hypo, under + rtOivat, tithenai, to place). In scientific procedure. a conjecture as to the explanation of any phenomenon. made pro visionally and used as a starting-point for fur ther investigation and theory. Thus in studying the motion of the moon Newton made the hypothesis that its divergence from the straight line was due to the same eause that brings an apple to the ground when released from the stem. He then proceeded to find whether the rate of fall in the two cases was expressible by some single formula. After years it was dis covered that this was the case. It is now quite generally admitted that an hypothesis, to have any value, should be based on some known law, and should be the conjectural extension of that law to a new sphere under investigation. Thus Newton worked from the known law of gravita tion on the surface of the earth. The hypothesis of a luminiferous ether is based on the known laws of the motion of fluids, etc. Such a known law is called a Vera causa. Some logical purists insist not only that the point of departure must be a known law, but that the extension of this law to another sphere may not be legitimately made un less its extension is at least conceivably verifiable by sense-perception. Thus they claim that the existence of the luminiferous ether is not a rigidly logical hypothesis, because the ether is thought of as having no properties perceivable by our senses without the use of instruments that no one even supposes capable of construction.
Such an interpretation of the rem rausa is, how ever, not in accordance with actual scientific practice, nor is it theoretically justifiable. It would rule out the hypothesis of the prevalence of gravitation even within the solar system, as no one ever did or presumably ever will perceive with any of his senses the action of gravita tion except as a movement, and this movement is the thing to he explained by the law of gravita tion, and not its further perceptual justification. An hypothesis is a provisional attempt to think things together as instances of the prevalence of the same law; and all that is necessary to make an hypothesis valuable is that it should furnish some conception which shall at least provisionally unify experience by reducing it to law. A distinction is often made between theory and hypothesis. A theory is said to be a verified hypothesis. This distinction is one of degree, not of kind. Even an hypothesis which has been `verified' may be overthrown by new facts; so that it is rather the fashion nowadays to speak of all the conceptions of natural science as `working hypotheses.' and to say that they are accepted only tentatively as the basis for further investigations and for further theorizing on re sults. As to whether there is any unconditional element in scientific conceptions, see KNOWLEDGE, THEORY OF. For bibliography of the subject, see the works cited in the article Lome.