69. IV. Other Accidental Forms of Demonstration. -- (1) Direct and indirect demonstration: This distinction is connected rather with extrinsic circumstances than with the nature of things. The direct demonstration (including all the forms thus far enumerated) shows, without any deviation, that the conclusion is virtually contained in the premises. The indirect demonstration, taking into account the subjective dispositions of the person to whom it is addressed, demands his positive adhesion to the truth of the conclusion through the rejection of the contradictory proposition. E. g.: The demonstration of free will by proving the absurdity of the consequences of determinism.

The indirect demonstration is also called demonstration by the impossible, or reductio ad absurdum.

(2) Absolute and relative, or ad hominem, demonstration.

(3) Aristotle also opposes to the scientific demonstration a demonstratio a signo or per signum, an extrinsic proof adduced from things exterior to that which is to be demonstrated. See example (74).

These accidental or secondary forms of demonstration may be reduced to the fundamental distinction between the demonstration hoti, and dioti.