Practical truth in Aristotle

Anthony Kenny

in Episteme, etc.

Published in print December 2011 | ISBN: 9780199696482
Published online May 2012 | e-ISBN: 9780191738036 | DOI:
Practical truth in Aristotle

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This chapter argues that Aristotle never succeeded in setting out rules for valid practical reasoning, though the De Motu Animalium makes clear that he fondly hoped that they would turn out to have a close resemblance to his theoretical syllogistic. That hope was delusory, because practical reasoning has a special feature: defeasibility. Theoretical reasoning is not defeasible: that is to say, the addition of a new premiss cannot invalidate a previously valid inference. A conclusion that follows from a set of premisses will follow from any larger set that includes them. The same is not true of practical reasoning. A course of action which may be reasonably estimated as good on the basis of a particular set of premisses may cease to be reasonable if further premisses (for example, about the unintended consequences of the action) are brought into the picture. This defeasibility of practical reasoning has prevented not only Aristotle, but every subsequent logician, from presenting a satisfactory formulation of practical inference.

Keywords: Aristotle; theory of practical truth; prohairesis; defeasibility

Chapter.  4294 words. 

Subjects: Ancient Philosophy

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