Now we have already seen in what the unity to which the many states belong must be taken to consist. We found that this unity is essentially teleological; that group of states, we saw, is one thing which functions as one in regard to an end or interest, or, as we may also say, is the embodiment of coherent structure. The same is true of the process of change. The earlier and later stages of the process are differences in an identity precisely because they constitute one process. And a process is one when it is the systematic realisation of a single coherent end. To be one process means to be the systematic expression in a succession of stages of a single coherent plan or law. The succession of stages is thus welded into a unity by the singleness of the plan or law which they embody, and it is this systematic connection of each stage with all the rest which we express by saying that whatever changes possesses an underlying permanent identity of character. It would amount to precisely the same thing if we said the successive states of anything that changes form a connected system.
We must be careful here, as we were in dealing with the problem of Substance, not to be misled by taking symbolic aids to imagination for philosophical truths. Just as it is easy to imagine the “substance” of things as a sort of material substratum, it is easy to imagine the identity which pervades all changes as that of a number of pieces of matter, and to think of the changes as constituted by their motion through space. But such a representation must not be taken for anything more than an aid to imagination. It helps us to make a mental diagram, but it throws absolutely no light upon the real nature of the connection between the identity and the succession. For the same problem breaks out within each of the “self-identical” pieces of matter; we have to say what we mean by calling it one and the same throughout the series of its changing positions, and the necessity of answering this question shows us at once that the identity of a material particle throughout its motion is only one case of that identity pervading succession which belongs to all change, and in no sense affords any explanation of the principle it illustrates.[[100]] As a recent writer puts it, “it seems to be a deeply rooted infirmity of the human mind ... that it can hardly conceive activities of any sort apart from material bases, ... through habitually seeking to represent all phenomena in mechanical terms, in terms of the motion of little bits of matter, many of us have come to believe that in so doing we describe the actual events underlying phenomena.”[[101]] This “disease of the intellect,” as the same writer aptly calls it, is nowhere more insidious than where we are dealing with the problem of Change.
Change, then, involves two aspects. It is a succession of events in time, and these events are connected by a systematic unity in such a way that they form the expression of a plan or law of structure. The series of successive states which make up the history of a thing are the expression of the thing’s nature or structure. To understand the thing’s structure is to possess the key to the succession of its states, to know on what principle each gives way to its successor. And similarly, to have complete insight into the nature or structure of Reality as a whole would be to understand the principles according to which every transitory event in the history of the Universe, regarded as a series of events in time, is followed by its own special successor.
It is evident that, in proportion as our knowledge of any thing or system of things approaches this insight into the laws of its structure, the processes of change acquire a new character for us. They lose their appearance of paradox, and tend to become the self-evident expression of the identity which is their underlying principle. Change, once reduced to law and apprehended as the embodiment in succession of a principle we understand, is no longer change as an unintelligible mystery. We should bear this in mind when we reflect on the doctrine of Plato that the physical world must be unreal because the scene of incessant change. Such a view is only to be understood by remembering that before the invention of the mathematical methods which have enabled us with such conspicuous success to reduce physical phenomena to orderly sequence according to law, the physical world necessarily appeared to the philosopher a scene of arbitrary change following no recognisable principle. Change, so far as understood in the light of its principle, has already ceased to be mere change.[[102]]
§ 3. Ground and Consequence. In the technical language of Logic, the underlying principle of any system is called its Ground, the detail in which the principle finds systematic expression is called its Consequence. Ground and Consequence are thus one and the same systematic whole, only considered from two different points of view. The Ground is the pervading common nature of the system, thought of as an identity pervading and determining the character of its detail; the Consequence is the same system, looked at from the point of view of the detail, as a plurality of differences pervaded and determined by an identical principle. The understanding of a process of change thus clearly consists in bringing it under the principle of Ground and Consequence. In so far as we are successful in detecting a principle in the apparently arbitrary succession of events, these events become for us a system with a common principle of structure for its Ground, and a plurality of successive states as its Consequence.
Change is not, however, the only instance of the principle of Ground and Consequence. These two aspects may also be found in systematic wholes which contain no element of succession in time, e.g. in a body of logical deductions from a few fundamental premisses. The special peculiarity of the case of Change is that it is the principle of Ground and Consequence as applied to a material which is successive in time. As thus applied, the principle has received the special name of the Principle of Sufficient Reason, and may be formulated thus: Nothing takes place unless there is a sufficient reason why it should occur rather than not. It is clear that such a proposition is a mere result of the application of the conception of Reality as a systematic whole to the special case of the existence of the successive in time. It is therefore simply one case of the fundamental axiom of all knowledge, the axiom that what truly exists is a coherent whole.[[103]] We must of course observe that the principle does nothing to solve the perhaps insoluble problem why succession in time should be a feature of experience. This is a question which could only be answered if we could show that succession in time is a logical consequence of the existence of any multiplicity forming a systematic whole. Until we are able to establish this result, we have simply to accept succession as a datum of our experience. (Yet for some light upon the problem, see infra, Bk. III chap. 4, § 9)
§ 4. Causality. So far we have said nothing of a concept which is much more familiar in the popular treatment of the problem of Change than that of Ground and Consequence, the concept of Cause. In proceeding to discuss this concept, it is necessary in the first place to explain which of the numerous senses of the word we are taking for examination. There was an old scholastic distinction, which still reappears occasionally in philosophical writings, between the Causa cognoscendi, or reason for affirming a truth, and the Causa existendi or fiendi, the cause of the occurrence of an event. It is this latter meaning of the word “cause,” the meaning which is predominant wherever the term is used in modern scientific language, that we shall have in view in the following sections.
The Causa cognoscendi, or logical reason for the affirmation of a truth, as distinguished from the psychological factors which lead a particular individual to affirm it, is clearly identical with what modern logicians call the Ground. A given proposition must logically be affirmed as true in the last resort, because it fills a place in a wider system of truths which no other proposition would fill. Thus, e.g., a special proposition about the relation between the sides and angles of a triangle is logically necessitated, because it is an integral element in the development of a system of geometrical ideas which repose as a whole upon certain fundamental assumptions as to the character of spatial order. The original presuppositions cannot be worked out to their logical consequence in a body of internally coherent geometrical notions unless the proposition in question is included in that body. And reciprocally, the logical justification for regarding these presuppositions rather than any others as sound, lies in the fact that they yield a body of internally consistent consequences. Incidentally, we see by means of this illustration that Ground and Consequence are mutually convertible, which is what we might have inferred from the way in which we defined them as mutually complementary aspects of a single systematic whole.
What we are concerned with in the everyday and scientific treatment of Causation, is not this purely logical relation of Ground and Consequence, but something partly identical with it, partly different. The Causa fiendi has no significance except in connection with occurrences or events in time, and may roughly be said to correspond with what Aristotle denotes the “Source of Change”—ἀρχὴ κινήσεως or ὅθεν ἡ κίνησις—and his mediæval followers named the Efficient Cause. Cause, in the popular sense of the word, denotes the attempt to carry out the principle of the interconnection of events in a system along special lines by regarding every event as completely determined by conditions which are themselves previous events. Widely as the popular and the scientific uses of the term “cause” diverge in minor respects, they agree in the essential point. That every event has its cause is understood, both in everyday life and in the sciences which use the concept of causation, to mean that the occurrence and the character of every event in the time-series is completely determined by preceding events. In more technical language, causation for everyday thought and for the sciences means one-sided dependence of the present on the past, and the future on the present.
It is, of course, obvious that the principle of Causation as thus understood is not a necessary logical deduction from the principle of Ground and Consequence. It might be the case that all occurrences form a coherent plan or system, such that if you once grasped the principle of the system you could infer from it what precise occurrence must take place at any one moment, and yet it might be impossible to discover this principle by an examination of the course of events up to the present moment. In other words, the principle of the systematic interconnection of events might be valid, and yet the events of the present might depend on those which will succeed them in the future no less than on those which have preceded them in the past. In that case it would be impossible with absolute logical certainty to infer what will occur at a given moment from the mere examination of what has preceded, i.e. the principle of Causation as used in the sciences would not be logically valid.[[104]]