It does, it may be replied, but whether the necessity which science assumes is the same as that maintained by the metaphysical theory in question, may be doubted. The metaphysical theory is that everything which happens happens of necessity, and could not have happened otherwise than it did. The assumptions which science makes with regard to causation are that nothing can happen unless the conditions requisite to its production are fulfilled, and that when those conditions are present the result necessarily follows. The question is whether this scientific necessity is the same as that metaphysical necessity; or, if they are not the same, whether either is a logical consequence from the other. They are not the same: the scientific assumption is hypothetical, the metaphysical absolute. The former says that things will happen in one way, if certain conditions are fulfilled, in another if they are not; the latter that they absolutely must happen in this way, and not in that; and that it is an illusion to imagine that they can happen _either_ this way _or_ that. Science allows us the alternative; the metaphysical theory declares that the alternative is an impossibility or an illusion. The metaphysical theory may be right, but it is not the same thing as the scientific assumption. Neither can it be exhibited as a logical presupposition of or consequence from the scientific assumption. From a hypothetical "if" you cannot logically get an absolute "must." It may be a scientific truth that, if an electric spark is passed through two atoms of hydrogen and one of oxygen, a drop of water will be formed. But it does not follow that therefore an electric spark must be passed through them.