Nor can it be said, thus far, that the laws of science lead us to the theory of necessity as their logical conclusion. It may be true that if I walk over a precipice I shall fall to the bottom, in accordance with the law of gravitation. But it does not logically follow that therefore I must walk over. It may be true that a suspension bridge will fall in the same way, if the supports be removed; but it does not follow that they are therefore bound to give way. It may be true that if nature is uniform certain sequences will happen; but it does not therefore follow that nature must be uniform. In other words, the theory of necessity, if true, cannot be based on science, but must rely on some metaphysical considerations. Science does not undertake to prove even that nature is uniform, much less that it is uniform of necessity. The opposite theory, that the uniformity of nature or of human nature is due to the action of a will freely manifesting itself as uniform, may be considered superfluous from the scientific point of view. But the theory of necessity from the same point of view is equally superfluous. As long as events do happen uniformly, science has all she wants—whether their uniformity is of will or of necessity is for her quite a superfluous question. And if science were all that man wanted, these rival metaphysical theories would be of no interest to him either. But the persistency of the attempt to extract some support for the metaphysical theory of necessity out of the facts of science shows that men of science, being men, must have their metaphysics.
Are there, then, other facts of science, or assumptions essential to science, which require the metaphysical theory of necessity as their presupposition or entail it as their natural consequence? Probably the reply will be that there is one such principle: that of the Universality of the Law of Causation. The assumption that everything must have a cause may be on the part of science a pure assumption, and one which, like the Uniformity of Nature, cannot be proved by science; but it does, it may be said, assume the existence of a necessity in things.
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.
It is obvious that the difference between science and metaphysics in the matter of necessity is that, whereas science cautiously says, "If certain conditions are fulfilled, certain results will ensue," metaphysics boldly says, "The conditions on which the whole future depends are already absolutely fixed." Once more, this metaphysical theory may be true; but, if so, it is not from science that it derives its truth. The transition from the "if" of science to the "must" of metaphysics is illogical, though not unnatural, and is facilitated by a certain amount of obscurity, which can be thrown over it by drawing illustrations from the past. Thus, if an event has already taken place, we may infer with certainty from the fact of its occurrence that the conditions necessary to produce it were realised. And as each of those conditions must have had a cause, we can infer again that the conditions requisite to produce them were fulfilled. And so we may travel back ad infinitum along a never-ending chain of cause and effect, always moving from one fixed and necessitated event to another event equally necessitated and fixed. Thus the whole past history of the universe may be exhibited as a necessary sequence of events; and the inference may be drawn, and for the purposes of the theory of metaphysical necessity must be drawn, that because the occurrence of an event proves that the conditions required for its production were realised, therefore they and they alone were bound to be realised. Yet this is simply our old familiar non sequitur thrown into the past tense. It is true that if I walk over a precipice I shall fall, according to the law of gravitation. But I am not therefore bound to walk over. It is true that the man who fell over the cliff obeyed the law of gravity. But we cannot infer either from the law of gravitation or from the fact of his falling that he was bound to fall. We can infer that the conditions requisite to produce the fall were present, but we cannot infer from the fall that they were bound to be present. It may be quite true that they were bound to be present, but the effect which followed on them cannot be alleged either as the cause or the proof of such necessity. We must look for the reason of the necessity—if there be any necessity in the case—elsewhere. Shall we, then, say that the conditions of the fall were themselves effects of prior causes, without which they would not have happened? That again is true, but the fact that Z would not have happened had not Y preceded, is not in itself any proof that Y was bound to happen. And so we may travel back ad infinitum along the never-ending chain of cause and effect without ever finding ourselves in a position to infer from the law that everything must have a cause, that this cause was bound to operate rather than that. The occurrence of Z is no proof that Y was bound to happen, nor is the fact that Y really happened any proof that its cause X was bound to occur—and so we may work back to the beginning of the alphabet. The fact that B took place shows that A actually occurred, but not that A, rather than A1 or A2, was bound to occur. And if A is the beginning, what was the nature of the necessity (prior to the beginning of things) which determined in favour of A rather than A1 or A2?
We may indeed say, if we like—since no one can prevent us from saying things without proof or probability—that the mere fact that A happened shows that it was bound to happen. But then we might just as well have said it of Z, and saved ourselves the trouble of going through so much alphabet to get so little result. We might just as well say that as the explosion or the accident did happen as a matter of fact, it could not possibly have been prevented: Z was bound to happen under the circumstances, therefore the circumstances could not have been altered; only one result was possible under the conditions, therefore no other conditions were possible.
Or—to go back to the beginning of the alphabet once more—we may say with science that we are content with the fact that A did happen, or, since science does not profess to take us back to an absolute beginning (force and matter being eternal and without beginning), let us say we may, like science, be content with the fact that K can be shown to have happened; but whether K, rather than K1 or K2, was bound to occur there is nothing in science to show. If we take up this, the scientific, attitude, two consequences follow. First, there is nothing in science to require or countenance the metaphysical theory of necessity. Next, what is true of K is equally true of L or M or Z. The fact that L or M or Z occurred proves that the conditions did combine in the way necessary to produce L or M or Z, not that they were bound to combine in that way and could not have combined so as to produce L1 or L2, or Z1 or Z2 or Z3.
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.