Liberum de ratione judicium. Ratio, as often in St Thomas, is ratio formalis, or logos, the specific notion rather than the object of the specific notion, which is also the object of definition. So immediately above, sub ratione boni. I need hardly add that every specific notion is also a general concept. Not until intellect has universalised the object of choice and viewed it as a generality, is the will free.
Of God and His Creatures: 2.48