<p>What is the role played by mechanisms in medical reasoning? In this paper, we provide an inferential study of the use of mechanisms in medical reasoning. Medical reasoning includes clinical reasoning and biomedical research reasoning. It is not conceived in terms of a specific form of inference, but as a complex form of reasoning involving abductions, deductions, and inductions. This methodology sheds a new light on the notion of mechanistic evidence, which has been the object of a regain of interest in recent literature, but which remains unclearly defined. We claim that “mechanistic evidence” is ambiguous: it can mean “evidence of a mechanism” or “mechanism as evidence”. Then, two main groups of medical reasoning can be identified: inference <i>to</i> mechanisms and inference <i>from</i> mechanisms. This serves as a basis to distinguish between factual evidence and the more pragmatic notion of evidence as reasons to accept a claim. In this context, we assess the role of mechanisms in the process of producing evidence.</p>