Full Idea
Voluntary motion possesses the intrinsic property of being in our power and of obeying us, and its obedience is not uncaused, for its nature is itself the cause of this.
Gist of Idea
Voluntary motion is intrinsically within our power, and this power is its cause
Source
report of Carneades (fragments/reports [c.174 BCE]) by M. Tullius Cicero - On Fate ('De fato') 11.25
Book Reference
Cicero: 'On Fate, Stoic Paradoxes, Oratory', ed/tr. Rackham,H. [Harvard Loeb 1942], p.221
A Reaction
To say that actions arise from our 'intrinsic power' is not much of an explanation, but it is still informative - that you should study the intrinsic powers of humans if you want to explain it.