A tentative supposition with regard to an unknown state of affairs, the truth of which is thereupon subject to investigation by any available method, either by logical deduction of consequences which may be checked against what is known, or by direct experimental investigation or discovery of facts not hitherto known and suggested by the hypothesis. A classical example is the nebular hypothesis with regard to the origin of the solar system. The “truth” of this hypothesis may be checked by its ability to explain the properties of the solar system as now observed.