 We shall devote the rest of this article to clarifying Kant’s solution and its relationship with Hume’s conception of causation. For now, we simply note an important difficulty Kant himself raises in the Prolegomena. Whereas the concept of causality is, for Kant, clearly a priori, he does not think that particular causal laws relating specific causes with specific effects are all synthetic a priori—and, if they are not a priori, how can they be necessary? Indeed, Kant illustrates this difficulty, in a footnote to § 22, with his own example of the sun warming a stone (4, 305; 58):