Dialectical reasoning, which goes back to Heraclitus and Protagoras, stresses an active role for the mind in which meanings are generated by opposing any term or proposition to its opposite. It serves as a base for the personality theories of Freud and Jung, which were anticipated by Adler in 1908 when he formulated the principle of the transformation of a drive into its opposites. Similarities and differences between Alder's and Freud's approaches to dialectical apperception and the problem of thesis-antithesis-synthesis are discussed. Included in the discussion are 'male-female', 'above-beneath', individual and society, and the resolution of conflicts by synthesis. The influence of Hegel is mentioned but, surprisingly, not that of Marx.