The mental model theory of reasoning postulates that individuals establish the consistency of a set of assertions by constructing a mental model in which all the assertions hold. Mental models represent what is true but not what is false, and this principle of ‘truth’ predicts that certain assertions should yield systematic errors. We report an experiment in which participants evaluated the consistency of assertions based on quantifiers and sentential connectives, e.g., All of the artists are barbers or else all of the barbers are artists; Some of the artists are not barbers. The results showed that participants judged consistent assertions to be inconsistent, and vice versa, much more often for the predicted assertions than for control problems, which should be unaffected by the failure to represent what is false. These results provide a litmus test for mental models, because no current alternative theories of reasoning predict them.