HILARY KORNBLITH IN DEFENSE OF DEDUCTIVE INFERENCE (Received 9 March 1994) Bill Lycan has offered us a big and controversial thesis: human beings do not and should not reason deductively.1 The more I think about this issue, the less convinced I am that Lycan is right. So what I'd like to do here is explain why I believe that human beings so frequently reason deductively, and moreover, why they should. I don't, of course, believe that deductive inference is the only kind of inference human beings do or should engage in; I don't believe that human beings do or should try to draw out all of the deductive consequences of their beliefs. But then again, no one has ever believed these things. What I do believe, and what is widely believed, is that deductive inference is an important part of human reasoning, both from a descriptive and from a normative perspective; and this, of course, is just what Lycan denies. As an illustration of the larger claim he wishes to make about deduc- tive inference, Lycan discusses the case of Modus Ponens. He argues that people do not, and should not, reason in accord with that rule. Since this rule is about as obviously valid as deductive rules get, and since people do seem to reason in accord with it, this is a particularly important case. If Lycan is right about Modus Ponens, then the case for a mental logic is, at a minimum, badly weakened. I do not believe, however, that Lycan is right about Modus Ponens. Although Lycan anticipates the kind of move I wish to make [see 236], and quickly rejects it, I want to spell out the case in favor of Modus Ponens, and with it mental logic, in a bit more detail. In the end, I have a number of worries about Lycan's position, both in points of detail and in overall conception. I think I understand what is involved in the research program of discovering a mental logic, but I am less clear about what would be involved if we were to sign on with Philosophical Studies 76- 247-257, 1994. © 1994 KluwerAcademic Publishers. Printed in the Netherlands.