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I came across Kant asserting the following in Metaphysik L₂:

Ab esse ad posse valet consequentia; a posse ad esse non valet consequentia.

This appears to be a very old precept that probably preceded Kant's time, so I'm looking for the originator or the earliest known reference to it.

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This seems to be a "basic principle of scholastic thought", referred as "possibility as potency". Regarding being related to scholastic thought, that is implied in texts like this one and this one. This 1787 book on theology and philosophy, published just two years after Kant's Grundlegung zur Metaphysik der Sitten, states (emphasis in original):

V. Quod existit, id est possibile : principium illud Scholasticorum verbis enuntiabimus: ab esse ad posse valet consequentia, seu quod perinde est: ab existentia ad possibilitatem valet consequentia: ...

Regarding the principle itself, according to this book, seems to refer to modal logic, which seems to emanate from Aristotle, and was further developed by William of Ockham and John Duns Scotus, two prominent scholastic authors. More about this principle in Aristotle's and Duns Scotus thought can be found here, here and here.

  • You're references were very helpful. Thanks! I didn't find John Duns Scotus saying that verbatim, but he said something which means essentially the same thing: "Sed malo de possibili proponere conclusiones et praemissas. Illis quippe de actu concessis, istae de possibili conceduntur; non e converso." – Expedito Bipes Oct 25 '18 at 13:42
  • @ExpeditoBipes You're welcome. Yes, I could not find that same expression either, but as far as I could get from the other links in the answer, no one seem to suggest Duns Scotus or Ockham stated the principle in those same words. – luchonacho Oct 25 '18 at 13:46

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