Full Idea
Everything that exists is either a substance or an accident.
Clarification
An 'accident' is a contingent quality of something
Gist of Idea
Everything that exists is either a substance or an accident
Source
Albert of Saxony (On 'Physics' [1357], I.18), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 13.2
Book Reference
Pasnau,Robert: 'Metaphysical Themes 1274-1671' [OUP 2011], p.252
A Reaction
This seems to be the view of those who base their ontology on first-order classical logic. The more austere reading of that makes the accidents into sets of substances, so it's just substances. All the non-substance stuff cries out for recognition.