Full Idea
No substance is understood in its own right, except in the most universal of concepts, namely of 'being'.
Gist of Idea
Substance is only grasped under the general heading of 'being'
Source
John Duns Scotus (In Metaphysics [1304], III n. 116), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 07.3
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.125
A Reaction
This is a fairly standard scholastic pessimism about knowing anything about substance. The modern view suggests that actually scientists know 'substance' pretty well.