We investigated whether a basic deductive inference, namely the modus ponens, is an automatic processing step carried out even in the absence of willingness to reason and awareness of all the premises. We used an exhaustive set of conditional problems concerning numbers, with a subliminal second premise, followed by a target number. Participants evaluated whether the target number was odd or even. The target number could match or not match a valid conclusion endorsed by the premises. We found that evaluations of target numbers matching the conclusion of a modus ponens were faster than the evaluations of numbers following all other types of problems. This finding shows that modus ponens is automatic and can be triggered by subliminal stimuli.