Hume points out, that the nature of the phenomena we consider can have nothing to do with the origin of the conception that they are connected by the relation of cause and effect. For that relation is nothing but an order of succession, which, so far as our experience goes, is invariable; and it is obvious that the nature of phenomena has nothing to do with their order. Whatever it is that leads us to seek for a cause for every event, in the case of the phenomena of the external world, compels us, with equal cogency, to seek it in that of the mind.
The only meaning of the law of causation, in the physical world, is, that it generalises universal experience of the order of that world; and, if experience shows a similar order to obtain among states of consciousness, the law of causation will properly express that order.
That such an order exists, however, is acknowledged by every sane man:
"Our idea, therefore, of necessity and causation, arises entirely from the uniformity observable in the operations of nature, where similar objects are constantly conjoined together, and the mind is determined by custom to infer the one from the appearance of the other. These two circumstances form the whole of that necessity which we ascribe to matter. Beyond the constant conjunction of similar objects and the consequent inference from one to the other, we have no notion of any necessity of connexion.
"If it appear, therefore, what all mankind have ever allowed, without any doubt or hesitation, that these two circumstances take place in the voluntary actions of men, and in the operations of mind, it must follow that all mankind have ever agreed in the doctrine of necessity, and that they have hitherto disputed merely from not understanding each other."—(IV. p. 97.)
But is this constant conjunction observable in human actions? A student of history could give but one answer to this question:
"Ambition, avarice, self-love, vanity, friendship, generosity, public spirit: these passions, mixed in various degrees, and distributed through society, have been, from the beginning of the world, and still are, the source of all the actions and enterprizes which have ever been observed among mankind. Would you know the sentiments, inclinations, and course of life of the Greeks and Romans? Study well the temper and actions of the French and English. You cannot be much mistaken in transferring to the former most of the observations which you have made with regard to the latter. Mankind are so much the same, in all times and places, that history informs us of nothing new or strange in this particular. Its chief use is only to discover the constant and universal principles of human nature, by showing men in all varieties of circumstances and situations, and furnishing us with materials from which we may form our observations, and become acquainted with the regular springs of human action and behaviour. These records of wars, intrigues, factions, and revolutions are so many collections of experiments, by which the politician or moral philosopher fixes the principles of his science, in the same manner as the physician or natural philosopher becomes acquainted with the nature of plants, minerals, and other external objects, by the experiments which he forms concerning them. Nor are the earth, air, water, and other elements examined by Aristotle and Hippocrates more like to those which at present lie under our observation, than the men described by Polybius and Tacitus are to those who now govern the world."—(IV. pp. 97-8.)
Hume proceeds to point out that the value set upon experience in the conduct of affairs, whether of business or of politics, involves the acknowledgment that we base our expectation of what men will do, upon our observation of what they have done; and, that we are as firmly convinced of the fixed order of thoughts as we are of that of things. And, if it be urged that human actions not unfrequently appear unaccountable and capricious, his reply is prompt:—
"I grant it possible to find some actions which seem to have no regular connexion with any known motives, and are exceptions to all the measures of conduct which have ever been established for the government of men. But if one could willingly know what judgment should be formed of such irregular and extraordinary actions, we may consider the sentiments commonly entertained with regard to those irregular events which appear in the course of nature, and the operations of external objects. All causes are not conjoined to their usual effects with like uniformity. An artificer, who handles only dead matter, may be disappointed in his aim, as well as the politician who directs the conduct of sensible and intelligent agents.
"The vulgar, who take things according to their first appearance, attribute the uncertainty of events to such an uncertainty in the causes as make the latter often fail of their usual influence, though they meet with no impediment to their operation. But philosophers, observing that, almost in every part of nature, there is contained a vast variety of springs and principles, which are hid, by reason of their minuteness or remoteness, find that it is at least possible the contrariety of events may not proceed from any contingency in the cause, but from the secret operation of contrary causes. This possibility is converted into certainty by further observation, when they remark that, upon an exact scrutiny, a contrariety of effects always betrays a contrariety of causes, and proceeds from their mutual opposition. A peasant can give no better reason for the stopping of any clock or watch, than to say that it does not commonly go right. But an artist easily perceives that the same force in the spring or pendulum has always the same influence on the wheels; but fails of its usual effect, perhaps by reason of a grain of dust, which puts a stop to the whole movement. From the observation of several parallel instances, philosophers form a maxim, that the connexion between all causes and effects is equally necessary, and that its seeming uncertainty in some instances proceeds from the secret opposition of contrary causes."—(IV. pp. 101-2.)
So with regard to human actions:—
"The internal principles and motives may operate in a uniform manner, notwithstanding these seeming irregularities; in the same manner as the winds, rains, clouds, and other variations of the weather are supposed to be governed by steady principles; though not easily discoverable by human sagacity and inquiry."—(IV. p. 103.)