Perhaps it may be said that the following is the proper way of stating the case: We have reason for believing that, as a matter of scientific necessity, if L is at work it can only produce M, and not M₁ or M₂ (the application of a light to a barrel of gunpowder can have only one result). But L was at work, therefore M alone could result. Quite true, but that does not show that the light was bound to be applied, or that the powder might not have been damp. In fine, the moment the conditions requisite for the explosion are combined, the explosion is necessary, M is the only possible result; but until then the explosion is not necessary, and the result may be M₁, or M₂, or M₃. A cause (_i.e._ the conditions combined) can only have one effect; but until it has that effect it is not the cause, and may never be. Pre-existent causes, which must inevitably produce predetermined effects, are figments of the metaphysical imagination. Conditions which may, and, subject to the trebly hypothetical laws of science, will combine in certain ways are scientific facts. In fine, the Uniformity of Nature, in the sense in which Nature is assumed, both by science and by common sense, to be uniform, simply amounts to the assumption that under the same conditions the same consequences will ensue. But this uniformity neither requires nor entails necessity. The very form chosen by science for the expression of scientific laws proclaims the fact: "_If_ bodies are free to move," "_if_ counteracting causes be absent," "a body _tends_ to move in the same straight line." Whatever necessity is introduced into the truths of science thus expressed is obviously imported from without, and is no part of science. We may, if we choose, read necessity into science, but there is no warrant in science for doing so. Science is absolutely without prejudice on this point. If everything that happens happens of necessity, the gravitation formula will receive no accession to its truth. If there be no necessity in the case, each and every truth of science remains valid as long as the same consequences do ensue in the same circumstances. Since, then, science observes an armed neutrality in this dispute, and is concerned only to guard that assumption of the uniformity of nature which is vital to her existence, we must turn elsewhere for a decision of the question.