The concepts of categorical diagnosis and conjectural diagnosis are introduced. It is argued that in diagnostic reasoning conjectural diagnosis plays a more important role than categorical diagnosis. Attention is called to the inevitable vagueness of clinical language and to the suitability of epistemic logic and fuzzy logic for diagnostic reasoning.

Two common medical-ethical axions, the health-maximizing axiom and the personhood-respecting axiom, are discussed. On the basis of a philosophical analysis of personhood and freedom of the will it is shown that these two axioms are incompatible. The rejection of the first axiom is suggested.

Moore and Hutchin''s [1] assertion that the application of classical logic as the method of diagnostic reasoning may result in three paradoxes is critically examined.