Abstract

One of the most important principles of old logic and Aristotle's philosophy is the principle of presupposition, based on which "the affirmation of an attribute for a subject is posterior to the affirmation of that subject." From among the important consequences of this principle, we can refer to the following two propositions:1. An affirmative proposition without a subject is false.2. A negative proposition without a subject is true.The following two propositions are in opposition to the principle of conversion by contradiction because of the following example:A. The general negative proposition of "no non-existent is non-existent" is true because of the lack of subject.B. The conversion by contradiction of this proposition, i.e. "some non-existents are not non-existent", is false. In this paper, some other examples of the principle of conversion by contradiction will be presented which are inconsistent with the second consequence of the principle of presupposition proposition. Since the principle of conversion by contradiction has two definitions, the inconsistency of the principle of presupposition with this principle will be explained based on both definitions and with the help of some related examples. Some solutions will also be provided in order to remove this inconsistency, and it will be shown that none of them will be helpful in accomplishing this task. Finally, the standpoint of modern logic in this regard will be evaluated.