Causality, or the theory of the relation between cause and effect, is one of the most intricate and important questions of philosophical doctrine. All scientific investigation is occupied with the search for the causes of given events, or for the effects of given causes, and with the generalisation of these into laws of nature. But the nature and ground of the relation between cause and effect are obscure and disputed.
The difficulty of the question is largely increased by the uncertain signification of the word cause. Thus the investigation into the cause of things, with which early Greek speculation was occupied, was really an inquiry for the ultimate constituent or element from which the variety of actual existence had proceeded; and from this inquiry the quest for a principle of change or development was only gradually distinguished. The first important step in the direction of clear discussion was made in Aristotle's doctrine of the 'four causes': the material cause, out of which a thing is framed; the formal cause, or the essence or idea of the thing; the efficient or active cause, by means of which it took its present form; and the final cause or purpose it subserves. These, it is to be observed, are not so much causes in the modern sense of the term, as principles which enter into the existence of everything. In modern science the meaning of the term is much more restricted, corresponding in some degree to what Aristotle called the efficient cause. Thus both Bacon and Descartes wished to banish the notion of final cause from the scientific interpretation of nature; and although, in Bacon's own method, science was treated as an inquiry into the form or true nature of things (corresponding thus to Aristotle's formal cause), this notion has had little influence. What Descartes sought, and what science still seeks, is the connection rather than the essence of things; and its ideal is a mechanical interpretation of nature in terms of matter and motion. In modern science cause may therefore be said to mean the explanation of change. To some extent it corresponds with Aristotle's efficient cause. But the notion of efficient cause has itself undergone a profound modification, which seems to have been carried out alongside of the formulating of the principle of the conservation of energy. The tendency in science has been to replace the notion of power or efficiency by that of order or constant sequence. The genesis and justification of the notion of efficiency are matters of dispute: whether it is an a priori intuition, or derived from the consciousness of the voluntary direction of attention, or from the sensations of innervation and muscular resistance. Both Berkeley and Hume directed a vigorous polemic against the doctrine of power expressed by Locke, as going beyond the observed facts of the motion of bodies, and Hume refused to see in mind any more than in matter anything else than a succession of impressions and ideas. Into the rights of this controversy it is impossible to enter here. But clearness of scientific statement has certainly been gained by the extrusion of the notion of power, and substitution for it of that of regular sequence. It is in following out this view of the physical as distinct from the efficient cause that the term comes to be defined as the aggregate of the conditions or antecedents necessary to the production of the effect: meaning by necessary conditions those conditions without which the effect either would not have existed at all, or would have been different from what it is. In popular language, however, and even in most scientific inquiries, the term cause is restricted to the one or two conditions by the intervention of which amongst other more permanent conditions the effect is produced. Thus it is noticeable that while the former or more complete definition corresponds with that expressly given in J. S. Mill's Logic, his inductive methods are entirely devoted to explaining modes of discovering causes in the narrower or popular signification.
It is in this meaning of the term that science investigates causes. In doing so, it goes on the presupposition that every event or change has a cause. This has been called the Law of Universal Causation, and may be expressed by saying that the explanation of every event is to be found in antecedent conditions. Scientific investigation also presupposes the Law of the Uniformity of Nature, that the same conditions or cause will be followed (at all times and places) by the same effects. The grounds and mutual relation of these two assumptions form the chief subject of controversy in the philosophical theory of causality. It is to Hume that the credit is due of having drawn attention to the difficulties involved in the principle of causation, in such a way as to determine the whole course of subsequent philosophy. All reasoning about matters of fact, he shows—all physical science, therefore—depends on the relation between cause and effect. Yet, between the cause and the effect there is no discoverable connection. 'There appears not, through all nature, any one instance of connection which is conceivable by us. All events seem entirely loose and separate. One event follows another; but we never can observe any tie between them. They seem conjoined, but never connected.' Hume's own solution of the difficulty is found in the law of mental association. 'The mind,' he says, 'is carried by habit upon the appearance of one event to expect its usual attendant, and to believe that it will exist. This connection, therefore, which we feel in the mind, or customary transition of the imagination from one object to its usual attendant, is the sentiment or impression from which we form the idea of power or necessary connection. Nothing further is in the case. . . . When we say, therefore, that one object is connected with another, we mean only that they have acquired a connection in our thoughts, and give rise to this inference by which they became proofs of one another's existence: a conclusion which is somewhat extraordinary, but which seems founded on sufficient evidence.' The conclusion to which Hume is driven is thus that, while all reasoning about matters of fact is founded on the principle of causality, this principle has itself no other basis than the mental tendency to pass from one impression to the idea of another impression previously experienced in conjunction with the former. Hume's solution is thus not sceptical (except as regards the application of causality or any other principle beyond experience), but it is subjective: the connection of things is resolved into a customary succession of ideas. Of the numerous theories of causation put forward since the question was thus opened, the two most important are J. S. Mill's rehabilitation of Hume's doctrine to suit the requirements of scientific investigation, and the opposed doctrine of Kant and his philosophical successors.
It is characteristic of Mill's doctrine that the principle of causality is made a consequence of the Law of the Uniformity of Nature: 'the familiar truth that invariability of succession is found by observation to obtain between every fact in nature and some other fact which has preceded it.' This principle, which is assumed in every scientific induction, is itself held to be the generalisation of a wide and uncontradicted experience.
A different position is given to the causal principle in Kant's philosophy. The Scottish philoso- phers and others, as well as Kant, had attempted replies to Hume, contending that causality is an intuitive judgment antecedent to experience. But such a reply remains an arbitrary assertion until it is shown how the causal judgment is connected with experience. In Kant's Critique of Pure Reason this connection is thoroughly investigated; the refutation of Hume is only part or consequence of a complete inquiry into the relation of reason to experience. It was, however, largely Hume's doctrine of causality that led to Kant's new point of view, and to the doctrine that experience is the product of the understanding, the realisation of its a priori forms. It is not the sequence of events in time, Kant holds, that gives rise to the principle of causality; but the pure notion of causality finds its realisation in this time-sequence, in which each event is determined by its antecedent. Kant's doctrine, as thus stated, is in full harmony with the principles and methods of modern science; asserting the principle that every change—i.e. each successive state—of the universe is the result of its preceding state, and at the same time leaving to empirical investigation the connection in experience of any one definite thing with any other.
The most important discussions of causality are those of Hume, Treatise of Human Nature, book i. part iii., and Essay Of the Idea of Necessary Connection; Kant, Critique of Pure Reason; and J. S. Mill, System of Logic, book iii. chaps. iii.–v. There is also elaborate treatment of the subject in the works of Reid, Stewart, and Hamilton. Dr Thomas Brown's Inquiry into the Relation of Cause and Effect contains much acute analytical thinking.