Full Idea
Necessity has no meaning other than the irresistible sequence of the effect where the cause is given. All necessity is thus conditioned, and absolute or unconditioned necessity is a contradiction in terms.
Gist of Idea
All necessity arises from causation, which is conditioned; there is no absolute or unconditioned necessity
Source
Arthur Schopenhauer (Abstract of 'The Fourfold Root' [1813], Ch.VIII)
Book Reference
Schopenhauer,Arthur: 'The World as Will and Idea', ed/tr. Berman,Jill and David [Everyman 1995], p.274
A Reaction
I.e. there is only natural necessity, and no such thing as metaphysical necessity. But what about logical necessity(e.g. 2+3=5)? I think there may be metaphysical necessity, but we can't know much about it, and we are over-confident in assessing it.