REDUCTIO QUESTIONS
Corcoran’s 2009 ARISTOTLE’S DEMONSTRATIVE LOGIC deals
decisively with several issues that had previously been handled by vague
speculation and dogmatic pontification if at all. One possible example:
Corcoran [2009, p. 13] proves conclusively that the imperfect syllogisms Baroco
and Bocardo—which Aristotle completed indirectly [by reductio-ad-impossible]—cannot
be completed directly. More generally, Corcoran shows that no valid premise-conclusion
argument, regardless of the number of premises, having an existential negative [“particular
negative” or “O-proposition”] as a premise can be completed using a direct
deduction—assuming of course that no premises are redundant and that the
conclusion is not among the premises. To be clear this means that for no such
argument is it possible to deduce the conclusion from the premises without using
reductio.
This result, called the EXISTENTIAL-NEGATIVE EXCLUSION [ENE],
was circulated informally by Corcoran much earlier but it seems not to have
been printed before 2009.
Q1: Was ENE, or a stronger result, stated in print before
2009?
Q2: Was ENE, or a stronger result, proved in print before
2009?
Q3: Are there other categorical arguments besides those
having existential negative premises that are not directly deducible?
Q4: What is a necessary and sufficient condition for a
categorical argument to be directly deducible?
Q5: Does Aristotle say anything about incomplete syllogisms
that cannot be completed directly?
Q6: Which later logicians say anything notable about
premise-conclusion arguments that cannot be shown to be valid by direct
deduction?
The above presupposes the system of deductions and the
definitions given in Corcoran’s 2009 ARISTOTLE’S DEMONSTRATIVE LOGIC.