Full Idea
Will's object is what is good, and so it cannot will anything but what is good.
Gist of Idea
The will can only want what it thinks is good
Source
Thomas Aquinas (Quaestiones Disputatae de Malo [1271], Q6.06)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.172