This paper seeks to show that logic and psychology imply each other and that syllogistic logic may legitimately be regarded as fundamental. This is hardly surprising since it was described 2000 years before the existence of any other kind was suspected. In the 19th century, psychology was concerned mainly with states of consciousness that, being evanescent and unique to the individual, could not form a foundation for logic. Logicians looked elsewhere, frequently to postulate sets, as a foundation for propositional logic from which predicate logic could be derived. Propositions, true or false but without descriptive content, may, however, be thought too abstract to furnish a satisfactory foundation; we are ordinarily acquainted only with propositions that have content-the kind with which syllogistic logic is concerned. Contemporary psychology and the logic of George Spencer Brown (1969) show, jointly, how logic may be implicit in behaviour, and account satisfactorily for the possibility of logical error in human problem solvers.