Ab esse ad posse valet, a posse ad esse non valet

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Ab esse ad posse valet, a posse ad esse non valet (consequentia) (also ab-esse-a-posse principle) is a key principle of philosophical logic. It says that a possibility (from being to being able) can be inferred from reality. But the other way of inferring reality from the possibility is invalid.

The logical rule expresses a modal consequence : "Quod existit, id est possibile" (Chr. WOLF, Ont. § 170). From the validity of the assertoric (Latin assertio 'claim') follows that of the problematic judgment, but not vice versa.

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  • Rudolf Eisler: Dictionary of Philosophical Terms. 1904
  • Friedrich Kirchner: Dictionary of basic philosophical terms. 1907
  • Immanuel Kant : Lectures on Metaphysics , p.41