Two experiments were carried out to make clear the mechanism of inferences and the representation of semantic categories with the instantiating information. In Experiment 1, verification times of instantiating-context-dependent category membership were measured for 24 graduate and undergraduate students. Verification times in RT (related true instance) condition were faster than those in UT (unrelated true instance). But those in RF (related false instance) condition were equal to those in UF (unrelated false instance). In verifying the context-dependent category, not forward association inferences from categories with the instantiating information to instance but backward association inferences from instances to instantiating contexts occurred. The results suggested that there weren't the instantiated representations of categories. In Experiment 2, categories with the instantiating information and instances were used as the predicate term in deductive reasoning. Judgements of validity and their reaction times under time pressure were measured for 24 graduate and undergraduate students. Percentages of valid responses were high in valid figure, and reaction times in universal negative proposition were longer. It was concluded that, when the tacit premises are introduced, backward association inferences are much more used than forward association inferences.