A hypothesis, it must be remembered, is any supposition, with or without evidence, made in order to deduce from it conclusions agreeable to known facts. If we succeed in doing this, we verify our hypothesis (unless, indeed, it can be shown that some other hypothesis will equally well suit these facts), and our hypothesis, when verified, ceases to be longer a hypothesis, takes its place as known truth, and in turn serves to explain those facts which would, on the supposition of its truth, follow from it as a cause. It is simply a short-hand process of arriving at conclusions in science. Suppose the problem to be the one already named--to prove that the central force of the solar system is one and the same with _gravity_. Now it may not be easy, or even possible in some cases, to establish the first step or premiss in such a chain of reasoning. The inductions leading to it may not be forthcoming. Hypothesis steps in and supplies the deficiency, by substituting in place of the induction a supposition. Assuming that distant bodies attract each other with a power inversely as the square of the distance, it proceeds on that supposition, and arrives at the desired conclusion. _In what Cases admissible._--Now this method is always allowable, and strictly scientific, whenever it is possible to verify our hypothesis, _i. e._, in every case in which it is possible to show that no law but the one assumed can lead to these same results; that no _other_ hypothesis can accord with the facts. In the case supposed, it would not be possible to prove that the same movements might not follow from some other law than the one supposed. It is not certain, therefore, that the moving force of the solar system is identical with gravitation, _merely because_ the latter would, if extended so far, produce the same results. In many other cases it _is_ practicable; indeed, _in all cases where the inquiry is not to ascertain the cause, but, the cause being already known, to ascertain the law of its action_. Even in cases where the inquiry is not of this nature, hypothesis is of use in the suggestion of future investigations, and, as such, is frequently indispensable.