Full Idea
There is necessarily for each existent thing a cause why it should exist.
Gist of Idea
There is necessarily for each existent thing a cause why it should exist
Source
Baruch de Spinoza (The Ethics [1675], I Pr 08 n2)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.50
A Reaction
The obvious response is 'how do you know that?' It has to the sort of a priori commitment we expect from a rationalist philosopher. It seems to me quite an appealing candidate for an axiom of human understanding.