 Critics of the cosmological argument contend that the Causal Principle or, where applicable, the broader Principle of Sufficient Reason (PSR) that underlies versions of the argument, is suspect. As Hume argued, there is no reason for thinking that the Causal Principle is true a priori, for we can conceive of events occurring without conceiving of their being caused, and what is conceivable is possible in reality (1748: IV). Neither can an argument for the application of the Causal Principle to the universe be drawn from inductive experience. Even if the Causal Principle applies to events in the world, we cannot extrapolate from the way the world works to the world as a whole (Mackie 1982: 85; Kant 1787: B638).