Full Idea
A thing is said to be impossible either because the essence of the thing itself or its definition involves a contradiction, or because no external cause exists determinate to the production of such a thing.
Gist of Idea
Things are impossible if they imply contradiction, or their production lacks an external cause
Source
Baruch de Spinoza (The Ethics [1675], I Pr 33)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.31
A Reaction
Is the contradiction in nature or in logic? How can he be sure that there doesn't exist some causeless thing?