This paper offers and account of the "system" of probable reasoning presented in Hume's Treatise and first Enquiry. The system is sceptical because it takes our beliefs to be the product of naturally occurring psychological mechanisms rather than logically sound judgment, and because it declares those beliefs to be ultimately unjustifiable. This paper explains how Hume was nonetheless able to provide for a logic of probable reasoning, grounded on natural, but unjustifiable beliefs.