Voluntas potest ab aliquo exteriori moveri quia indiget moveri ab alio sicut a primo movente.
The will can be moved by something exterior because it needs to be moved by another as first mover.
De ratione voluntarii est quod principium eius sit intra, sed non oportet quod hoc principium intrinsecum sit primum principium non motum ab alio. Unde motus voluntarius etsi habeat principium proximum intrinsecum, tamen principium primum est ab extra.
The formal aspect of the voluntary act is that its principle be within the agent, but it is not necessary that this inward principle be the first principle unmoved by another. Wherefore though the voluntary act has an inward proximate principle, nevertheless its first principle is from without.
Omne enim quod quandoque est agens in actu et quandoque in potentia, indiget moveri ab aliquo movente. Manifestum est autem quod voluntas incipit velle aliquid, cum hoc prius non vellet. Necesse est ergo quod ab aliquo moveatur ad volendum.
For everything that is at one time an agent actually, and at another time an agent in potentiality, needs to be moved by a mover. Now it is evident that the will begins to will something, whereas previously it did not will it. Therefore it must, of necessity, be moved by something to will it.
Hoc autem non est procedere in infinitum. Unde necesse est ponere quod in primum motum voluntatis voluntas prodeat ex instinctu alicuius exterioris moventis, ut Aristoteles concludit in quodam capitulo Ethicae Eudemicae.
But this process could not go on to infinity. Wherefore we must, of necessity, suppose that the will advanced to its first movement in virtue of the instigation of some exterior mover, as Aristotle concludes in a chapter of the Eudemian Ethics (vii, 14).