A subject S's belief that Q is well-grounded if and only if it is based on a reason of S that gives S propositional justification for Q. Depending on the nature of S's reason, the process whereby S bases her belief that Q on it can vary. If S's reason is non-doxastic––like an experience that Q or a testimony that Q––S will need to form the belief that Q as a spontaneous and immediate response to that reason. If S's reason is doxastic––like a belief that P––S will need to infer her belief that Q from it. The distinction between these two ways in which S's beliefs can be based on S's reasons is widely presupposed in current epistemology but––we argue in this paper––is not exhaustive. We give examples of quite ordinary situations in which a well-grounded belief of S appears to be based on S's reasons in neither of the ways described above. To accommodate these recalcitrant cases, we introduce the notion of enthymematic inference and defend the thesis that S can base a belief that Q on doxastic reasons P1, P2, …, Pn via inferring enthymematically Q from P1, P2, …, Pn.