Full Idea
If in the proposition 'This is an angel' subject and predicate stand for the same thing, the proposition is true.
Gist of Idea
A proposition is true if its subject and predicate stand for the same thing
Source
William of Ockham (Summa totius logicae [1323], II.c.ii)
Book Reference
Ockham,William of: 'Ockham's Philosophical Writings', ed/tr. Boehner,P [Hackett 1990], p.76
A Reaction
An interesting statement of what looks like a correspondence theory, employing the idea that both the subject and the predicate have a reference. I think Frege would say that 'x is an angel' is unsaturated, and so lacks reference.