 For Kant, the most significant rival theory of mental processing is that of his target, Hume. Hume concurs that a theory of experience requires an account of the processing of mental items, but he denies that such an account demands a priori concepts or issues in their legitimate applicability to experience. According to his theory, associationism, our mental repertoire consists solely of perceptions, all of which are sensory items – the more vivid impressions, which constitute sensory experience, and their less vivid copies, the ideas, which function in imagination, memory, reasoning, and conceptualization (1748, §§2, 3). Association is the process by which these perceptions are related and ordered. A signature feature of association on Hume’s view is that it requires no resources distinct from the perceptions themselves. How perceptions are ordered is solely a function of what perceptions alone can provide. Significantly, a subject not constituted solely of perceptions has no role in Hume’s theory; the Humean subject is just a collection of perceptions (1739: I, IV, vi). These last two features in particular make Humean associationism a highly economical theory, which provides it with an initial advantage over Kant’s more complex view. However, Kant contends that associationism cannot account for the facts to which the premises of the Transcendental Deduction appeal, and that synthesis by a priori concepts, that is, the categories, is required in addition.