Mary Shepherd assigns reason a central role in her philosophical system, and so to understand that system we must understand her conception of reason. Does she, like Hume, take reason to be a mere matter of factual process that operates over independently contentful representations? Does she, like Descartes, take it to be a process that is intended to track the rational relations among such representations? Or does she, like Kant, take reason to be a structural feature of representations without which (...) they would not have content at all? I argue that Shepherd's understanding of reason is broadly Kantian. Reason is a causal process whereby non-representational sensible qualities are combined in order to form representations of objects as loci of causal powers. These representations represent their objects as the external, continuously and independently existing, causes of the sensible qualities from which they have been constructed, and also as having causal powers to combine with other objects to have other effects. These representations are not only the result of a process of reasoning, but are also constituted by the rational relations into which reason puts them. Reason is both the causal process that forms such representations, and the structure of those representations themselves. (shrink)