ABSTRACT
Thinking is governed by abstract schemas. Verbal protocols illustrate spontaneous use, by logically unsophisticated subjects, of the schema known as modus tollens. The tollens inference schema appeared embedded within two reasoning strategies, the classical reductio ad absurdum and reasoning by elimination. The psychological reality of modus tollens is implicitly assumed by many theories in cognitive science and the hypothesis that it is a basic component of human cognition cannot be dismissed.
