Educational theory is routinely cited as justification for practice in medical education, even though the justification for the theory itself is unclear. Problem-based learning (PBL), for example, is said to be based on powerful educational principles that should result in strong effects on learning and performance. But research over the past 20 years has produced little convincing evidence for the educational effectiveness of PBL, which naturally raises doubts about the underlying theory. This essay reflects on educational theory, in particular cognitive theory, and concludes that the theory is little more than metaphor, not rigorous, tested, confirmed scientific theory. This metaphor/theory may lead to ideas for basic and applied research, which in turn may facilitate the development of theory. In the meantime, however, the theory cannot be trusted to determine practice in medical education. Despite the intuitive appeal of educational theory, medical educators have a responsibility to set aside their enthusiasm and make it clear to medical school faculty and administrators that educational innovations and practice claims are, at best, founded on conjecture, not on evidence-based science.