Full Idea
The will can avoid actually willing something by avoiding thinking of it, since mental activity is subject to will. In this respect we aren't compelled to will even total happiness, which is the only perfect good.
Gist of Idea
We don't have to will even perfect good, because we can choose not to think of it
Source
Thomas Aquinas (Quaestiones Disputatae de Malo [1271], Q6.h to 07)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.181