 Hume’s potentially idealist approach to causation is clearly on view in his 1748 Enquiry concerning Human Knowledge, which was quickly translated into German and would eventually provide Kant with the stimulus for his own aprioristic rather than empiricist argument for idealism with regard not only to causation but to all of what he called the categories of pure reason, including especially substance and interaction as well as causation. But since Kant was not familiar with the contents of Hume’s earlier Treatise of Human Nature, he did not know that Hume too had generalized his approach to causality to the cases of mind and body, nor did he know that Hume may have tried to sidestep Berkeley’s commitment to substances but not his idealism altogether by his theory of both minds and bodies as bundles of perceptions. Kant would try to avert Berkeley’s version of idealism by a different stratagem, but before we come to that we must consider Hume’s position more fully. Hume accepted from Locke and Descartes before him that the immediate objects of consciousness are what they had called ideas, although he reserves that word for copies or subsequently recalled perceptions rather than the originally experienced perceptions that he calls impressions. He also adopts the view of his predecessors that knowledge lies in the recognition of relations among impressions, ideas, or both, and divides those relations into two kinds, philosophical and natural. Philosophical relations are those immediately evident on reflection on or comparison of particular ideas, and include resemblance, identity, spatial and temporal relations such as above and below or before and after, number and degree, and logical contrariety (Hume 1739–40: I.I.5), while natural relations are those that are not immediately evident on reflection on a single impression or idea or in a single comparison of any number, but which instead become evident, or more properly are formed, only through repeated experience. Hume’s best known argument is then that causation is not a philosophical but a natural relation: the causal relation is comprised by temporal succession, spatial contiguity, and necessary connection, and while the first two are philosophical relations that are immediately apparent, the necessary connection between different ideas—those of a cause and its separate effect—is, unlike the necessary identity of two qualitatively similar ideas, not immediately apparent, as Hume puts it, to reason (1739–40: I.III.2), but instead grows only out of repeated experience, the repeated experience of qualitatively similar pairs of impressions which causes them to become linked in the mind, as we would ordinarily say, or at least in consciousness, as the careful Hume should say at most (1739–40: I.III.6). In fact, Hume’s argument is that repeated experience itself has two effects: it creates a habit of thought such that upon the presentation of an impression of one kind that has repeatedly been experienced in spatial and temporal conjunction with one of another kind, a vivid version of the idea of the kind of impression with which the first kind of impression has been repeatedly associated immediately occurs—this is the essence of causal inference or belief, because a belief is nothing but an idea that is almost as vivid and forceful as the impression of which it was once a copy (1739–40: I.III.7–8; this is the thought Hume could have learned from Arthur Collier)—and further, there is an actual feeling of the mind (as we would ordinarily say) being tugged from the one impression to the other idea—this is the basis of the idea of necessary connection, a connection which the mind then “spreads” upon its objects to form the idea of a necessary connection among them or their states (1739–40: I.III.14).