Full Idea
Reasoning powers can entertain opposite objects. Now will is a reasoning power, so will can entertain opposites and is not compelled to embrace one of them.
Gist of Idea
Since will is a reasoning power, it can entertain opposites, so it is not compelled to embrace one of them
Source
Thomas Aquinas (Quaestiones Disputatae de Malo [1271], Q6.x2)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.175