Kant stated that his recollection of David Hume awoke him from his dogmatic slumber. He was probably familiar with the translation of Hume's Enquiry Concerning Human Understanding (1739).
Hume argued against the a priori justification of causal judgments on the grounds that it is impossible to foretell how objects have behaved prior to having experienced them. One would have to invent an effect, and such invention is arbitrary. "The mind can never possibly find the effect in the supposed cause, by the most accurate scrutiny and examination. For the effect is totally different from the cause, and can never be discovered in it. . . . And as the first imagination or invention of a particular effect, in all natural operations, is arbitrary, where we consult not experience; so must we also esteem the supposed tie or connexion between cause and effect, which binds them together, and renders it impossible that any other effect could result from the operation of that cause" (25). Next, Hume launched a two-pronged attack against the a posteriori justification of causal inference. The two arguments are directed respectively against the universality and the necessity of judgments concerning causal connections. (This distinction accords well with Kant's distinction in the Aesthetic between two marks of the a priori.)
The first prong is the celebrated attack on the justification of induction. We are never entitled to infer from the fact that a certain pattern of succession has always been observed to the conclusion that the pattern holds in all cases, including those which are unobserved. To do so requires the assumption of the uniformity of nature, which itself is an inductive generalization standing in need of justification. "It is impossible . . . that any arguments from experience can prove this resemblance of the past to the future; since all these arguments are founded upon the supposition of that resemblance. Let the course of things be allowed hitherto ever so regular; that alone, without some new argument or inference, proves not that, for the future, it will continue to do so" (32).
The second prong is directed against necessity. Hume claimed that the only necessary connections of which human beings have any knowledge are those found in mathematics. In all "matters of fact," the only connection we can find is made in the imagination. After repeated exposure to a pattern of succession, we expect the customary pattern to repeat itself yet again. There arises a feeling of connection, which is the origin of the idea of a necessary connection.
"When one particular species of event has always, in all instances, been conjoined with another, we make no longer any scruple of foretelling one upon the appearance of the other, and of employing that reasoning, which can alone assure us of any matter of fact or existence. We then call the one object, Cause; the other Effect. We suppose that there is some connection between them; some power in the one, by which it infallibly produces the other, and operates with the greatest certainty and strongest necessity" (59).
Hume's argument is as follows. As noted above, one can never observe in a single instance what event will follow what other event, given that one has never experienced the first before. This can be seen experimentally, in cases in which a person is confronted with something entirely new to him. After experiencing a repeated pattern of succession of kinds of events, we predict that the pattern will repeat itself at every future time. But there is no difference, in our perception of the events themselves, between the first observation and any of the later ones, except that they have repeated themselves. We can find nothing in the events which can be called "cause," "power," "force," or what have you. So our expectation must have its origin in a customary transition in the mind.
Thus we must replace the objective definition of cause, "an object, followed by another, and where all objects similar to the first are followed by objects similar to the second" with a subjective definition "an object followed by another, and whose appearance always conveys the thought to that other" (60). To Kant, this is tantamount to making causality an illusion.
In the Prolegomena, Kant stated that Hume "justly maintains that we cannot comprehend by reason the possibility of causality, that is, of the reference of the existence of one thing to the existence of another which is necessitated by the former. . . . But I am very far from holding these concepts to be derived merely from experience, and the necessity represented in them to be imaginary and a mere illusion produced in us by long habit. On the contrary, I have amply shown that they and the principles derived from them are firmly established a priori before all experience and have their undoubted objective value, though only with regard to experience" (310). The proof is found in the Second Analogy in the Critique.
The Introduction to the Prolegomena contains some of Kant's more memorable remarks about a philosopher. He decribed the concept of cause, on Hume's treatment, as "a bastard of the imagination, impregnated by experience" (258). He found the consequences of Hume's arguments to be altogether destructive of metaphysics, and accused Hume of overlooking "the positive injury which results if reason be deprived of its most important prospects" (259n). Though "a great thinker," Hume was "hasty and mistaken" in his argument (258).
Kant even indulged in speculation about how Hume's thinking would have been different had he recognized that mathematical judgments are not only a priori but synthetic. "The good company into which metaphysics would thus have been brought would have saved it from the danger of a contemptuous ill-treatment, for the thrust intended for it must have reached mathematics, which was not and could not have been Hume's intention. Thus that acute man would have been led into considerations which must needs be similar to those that now occupy us, but which would have gained inestimably by his inimtably elegant style" (272).
