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Full Idea
Schopenhauer said the principle of sufficient reason is not susceptible to proof for the simple reason that it is presupposed in any argument or proof.
Gist of Idea
Sufficient Reason can't be proved, because all proof presupposes it
Source
report of Arthur Schopenhauer (Fourfold Root of Princ of Sufficient Reason [1813], §14 p.32-3) by Peter B. Lewis - Schopenhauer 3
Book Ref
Lewis, Peter B.: 'Schopenhauer' [Reaktion Books 2012], p.73
A Reaction
I would have thought it might be disproved by a counterexample, such as the Gödel sentence of his incompleteness proof, or quantum effects which seem to elude causation. Personally I believe the principle, which I see as the first axiom of philosophy.