Full Idea
The will is compelled by its ultimate goal (to achieve happiness), but not by the means to achieve it.
Gist of Idea
The will must aim at happiness, but can choose the means
Source
Thomas Aquinas (Quaestiones Disputatae de Malo [1271], Q6.07)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.173