Single Idea 3642

[catalogued under 10. Modality / D. Knowledge of Modality / 4. Conceivable as Possible / b. Conceivable but impossible]

Full Idea

You can't reason 'I know the triangle is right-angled, but I doubt Pythagoras' Theorem, therefore it does not belong to the essence of right-angled triangles that the square on the hypotenuse is equal to the sum of the squares on the other two sides'.

Gist of Idea

Pythagoras' Theorem doesn't cease to be part of the essence of triangles just because we doubt it

Source

comment on René Descartes (Meditations [1641], §6.78) by Antoine Arnauld - Objections to 'Meditations' (Fourth) 202

Book Reference

Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.109