Full Idea
The Principle may be stated as 'There is nothing without a reason why it should be rather than not be', which is a generalisation of the assumption which justifies the question 'Why?', which is the mother of all science.
Gist of Idea
'There is nothing without a reason why it should be rather than not be' (a generalisation of 'Why?')
Source
Arthur Schopenhauer (Abstract of 'The Fourfold Root' [1813], Ch.I)
Book Reference
Schopenhauer,Arthur: 'The World as Will and Idea', ed/tr. Berman,Jill and David [Everyman 1995], p.268
A Reaction
This faith is the core of philosophy, to be maintained against all defeatists like Wittgenstein and Colin McGinn. Reality must be rational, or we wouldn't be here to think about it. (Maybe!)