Full Idea
By substance I mean that which is in itself, and is conceived through itself; in other words, that of which a conception can be formed independently of any other conception.
Gist of Idea
Substance is that of which an independent conception can be formed
Source
Baruch de Spinoza (The Ethics [1675], I Def 3)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.45
A Reaction
A striking blurring of epistemology and ontology. He eventually settles for it being a concept rather than a fact of nature. It still begs a thousand questions, but it probably leads to monads and logical atoms.