diallelon (from ancient Greek di allelon, ‘through one another’), a circular definition. A definition is circular provided either the definiendum occurs in the definiens, as in ‘Law is a lawful command’, or a first term is defined by means of a second term, which in turn is defined by the first term, as in ‘Law is the expressed wish of a ruler, and a ruler is one who establishes laws.’ A diallelus is a circular argument: an attempt to establish a conclusion by a premise that cannot be known unless the conclusion is known in the first place. Descartes, e.g., argued: I clearly and distinctly perceive that God exists, and what I clearly and distinctly perceive is true. Therefore, God exists. To justify the premise that clear and distinct perceptions are true, however, he appealed to his knowledge of God’s existence. See also CIRCULAR REASONING , DEFINI- TIO. M.St.