 In Book II of the Groundwork (1785 [1996]) Kant claims the fundamental moral truths are synthetic a priori because moral truths are prescriptive. Kant held that the categorical imperative is not analytic, because although Kant thought the applicability of the categorical imperative to any given individual is deducible from the assumption that the individual is rational, the concept of the categorical imperative is not contained in the concept of a rational being. Kant thought the categorical imperative must be discovered a priori—through reason—because, as a fundamental moral law applying to all rational beings, it cannot be discovered through mere experience: one cannot learn how one should act from how people do act.