Full Idea
When things are so related that one causes the other to exist, the cause can exist without what it causes but not vice versa.
Gist of Idea
A cause can exist without its effect, but the effect cannot exist without its cause
Source
Thomas Aquinas (De Ente et Essentia (Being and Essence) [1267], p.103)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.103
A Reaction
This is open to question, if causes are supposed to be sufficient for effects. Presumably Aquinas would support the view that if the cause had not been, the effect would not have happened. But the current idea indicates the priority relation.