Full Idea
For Schopenauer there are physical necessity, logical necessity, mathematical necessity and moral necessity.
Gist of Idea
Necessity is physical, logical, mathematical or moral
Source
report of Arthur Schopenhauer (Fourfold Root of Princ of Sufficient Reason [1813]) by Christopher Janaway - Schopenhauer 2 'Fourfold'
Book Reference
Janaway,Christopher: 'Schopenhauer' [OUP 2002], p.20
A Reaction
These derive from four modes of explanation, by causes, by grounding in truths or facts, by mathematical reality, and by motives. Not clear why mathematics gets its own necessity. I like metaphysics derived from explanations, though. Necessity makers.