Full Idea
Nothing can be willed except good, but many and various things are good, and you can't conclude from this that wills are compelled to choose this or that one.
Gist of Idea
Nothing can be willed except what is good, but good is very varied, and so choices are unpredictable
Source
Thomas Aquinas (Quaestiones Disputatae de Malo [1271], Q6.h to 05)
Book Reference
Aquinas,Thomas: 'Selected Philosophical Writings', ed/tr. McDermott,Timothy [OUP 1993], p.181