<p>Logical reasoning is often presumed to require effortful cognition. However, there is evidence that reasoners may have access to a form of intuition that relies on variations in the availability of semantically related information. This may provide intuitive cues for judgments of conclusion validity or invalidity, allowing for very rapid forms of reasoning that do not rely on belief-based heuristics. We examined very rapid reasoning with formally identical syllogisms where the major premise was characterized by having relative Few or Many alternative antecedents. In four studies, we examined premises that had identical formal structure and syntax, with minimal differences in content, using two forms of invalid inferences: (“All A are B. X is B. X is A.”; “All A are B. X is not A. X is not B.”), and one valid form (“All A are B. X is not B. X is not A.”). All studies showed that under severe time constraint reasoners judged that the putative conclusion was invalid more often for premises with Many alternative antecedents than for those with Few alternatives. These results support the conclusion that semantically activated information can serve as a cue for intuitive, rapid inferences.</p>