Full Idea
If a thing can be conceived as non-existing, its essence does not involve existence.
Gist of Idea
If a thing can be conceived as non-existing, its essence does not involve existence
Source
Baruch de Spinoza (The Ethics [1675], I Ax 7)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.46
A Reaction
This points straight at the modern question of whether conceivability is a sufficient test for possibility. Personally I am close to Hume on this one. Necessary existence may not be ridiculous, but it is beyond human capacity to assert its occurrence.