Abstract: An a priori lie is a lie that conflicts with an a priori truth. Rather sportingly, the liar leaves himself open to refutation by armchair methods such as calculation. My thesis is that Immanuel Kant precludes the existence of a priori lies. For asserting a proposition requires raising a rational expectation of its truth. If the hearer believes the negation of an a priori proposition, Kant blames the deceived, not the deceiver. We are sometimes permitted to believe beyond the evidence but never against the evidence. The impossibility of a priori lies vindicates the advocate’s assumption that one cannot lie with deliberately invalid deductions or by engaging in insincere legal semantics.