Five types of psychoanalytic propositions have been distinguished. The first three describe empirical data, namely, (1) observable patterns of overt behavior; (2) introspectively observable patterns of consciousness; (3) unobservable, inferable unconscious processes. A variety of research methods is applicable to prove or disprove these propositions, provided these methods develop a sufficiently sophisticated research design and measure what they intend to measure.

No direct empirical evidence is possible in regard to the fourth category of propositions, i.e., to psychoanalytic theory. A theory is tested by its inner consistency, agreement with empirical data, and usefulness. Theory is not a statement of fact and can be modified as research progresses.