(3) The basic truth of demonstrative syllogism is the universal
immediate premiss, and the universal premiss asserts in affirmative
demonstration and in negative denies: and the affirmative
proposition is prior to and better known than the negative (since
affirmation explains denial and is prior to denial, just as being is
prior to not-being). It follows that the basic premiss of
affirmative demonstration is superior to that of negative
demonstration, and the demonstration which uses superior basic
premisses is superior.
(4) Affirmative demonstration is more of the nature of a basic
form of proof, because it is a sine qua non of negative demonstration.
26
Since affirmative demonstration is superior to negative, it is
clearly superior also to reductio ad impossibile. We must first make
certain what is the difference between negative demonstration and
reductio ad impossibile. Let us suppose that no B is A, and that all C
is B: the conclusion necessarily follows that no C is A. If these
premisses are assumed, therefore, the negative demonstration that no C
is A is direct. Reductio ad impossibile, on the other hand, proceeds
as follows.
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