_Liable to mislead._--It must be confessed, however, that it is a species of reasoning likely to mislead in many cases. Its chief value lies not in proving a position, but in rebutting objections; it is good, not for assault, but defence. As thus used it is a powerful weapon in the hands of a skilful master. Such it was in Butler's hands. § IV.--USE OF HYPOTHESES AND THEORIES IN REASONING. _Theory, what._--The terms hypothesis and theory are often used interchangeably and loosely. Confusion is the result. It is difficult to define them accurately. _Theory_ (from the Greek, [Greek: Theôria]; Latin, theoria; French, théorie; Italian, teoria; from [Greek: Theôreô], to perceive, see, contemplate) denotes properly any philosophical explanation of phenomena, any connected arrangement and statement of facts according to their bearing on some real or imaginary law. The facts, the phenomena, once known, proved, rest on independent evidence. Theory takes survey of them as such, with special reference to the law which governs and connects them, whether that law be also known or merely conjectured. _Hypothesis, what._--_Hypothesis_ ([Greek: hypo-tithêmi]) denotes a gratuitous supposition or conjecture, in _the absence_ of all positive knowledge as to what the law is that governs and connects the observed phenomena, or as to the cause which will account for them. _Theory may or may not be Hypothesis._--Hypothesis is, in its nature, conjectural, and therefore uncertain; has its degrees of probability--no certainty. The moment the thing supposed is proved true, or verified, if it ever is, it ceases to be hypothesis. Theory, however, is not necessarily a matter of uncertainty. After the law or the cause is ascertained, fully known, and no longer a hypothesis at all, there may be still a theory about it; a survey of the facts and phenomena, as they stand affected by that law, or as accounted for by that cause. The motion of the planets in elliptical orbits, was originally matter of conjecture, of hypothesis. It is still matter of theory.