Full Idea
You assert that the notion of substance is formed from concepts, and not from things. But are not concepts themselves formed from things?
Gist of Idea
Our notions may be formed from concepts, but concepts are formed from things
Source
Gottfried Leibniz (Letters to Burcher De Volder [1706], 1699.06.23), quoted by David Wiggins - Sameness and Substance Renewed 5.7
Book Reference
Wiggins,David: 'Sameness and Substance Renewed' [CUP 2001], p.154
A Reaction
A nice remark, which is true even of highly abstruse, abstract or fanciful concepts. You are still left with the question of how far away from reality you have moved when you construct things from your reality-based concepts.