Full Idea
There are four forms of explanation, depending on their topic. Causes explain objects. Grounding explains concepts, Points and moments explain mathematics. Motives explain ethics.
Gist of Idea
The four explanations: objects by causes, concepts by ground, maths by spacetime, ethics by motive
Source
report of Arthur Schopenhauer (Fourfold Root of Princ of Sufficient Reason [1813], §43 p.214) by Peter B. Lewis - Schopenhauer 3
Book Reference
Lewis, Peter B.: 'Schopenhauer' [Reaktion Books 2012], p.72
A Reaction
[My compression of Lewis's summary. I'm particularly pleased with this. I have done Schopenhauer a huge favour, should anyone ever visit this website]. The quirky account of mathematics derives from Kant. I greatly admire this whole idea.