Intentio proprie est actus voluntatis quia voluntas movet omnes alias vires animae ad finem.
Intention, properly speaking, is an act of the will because intention belongs first and principally to that which moves to the end, and the will moves all the other powers of the soul to the end (q9 a1).
Intentio est actus voluntatis respectu finis. Sed voluntas respicit finem tripliciter. Uno modo, absolute, et sic dicitur voluntas, prout absolute volumus vel sanitatem, vel si quid aliud est huiusmodi. Alio modo consideratur finis secundum quod in eo quiescitur, et hoc modo fruitio respicit finem. Tertio modo consideratur finis secundum quod est terminus alicuius quod in ipsum ordinatur, et sic intentio respicit finem. Non enim solum ex hoc intendere dicimur sanitatem, quia volumus eam, sed quia volumus ad eam per aliquid aliud pervenire.
Intention is an act of the will in regard to the end. Now the will stands in a threefold relation to the end. First, absolutely; and thus we have "volition," whereby we will absolutely to have health, and so forth. Secondly, it considers the end, as its place of rest; and thus "enjoyment" regards the end. Thirdly, it considers the end as the term towards which something is ordained; and thus "intention" regards the end. For when we speak of intending to have health, we mean not only that we have it, but that we will have it by means of something else.
Voluntas quidem non ordinat, sed tamen in aliquid tendit secundum ordinem rationis. Unde hoc nomen intentio nominat actum voluntatis, praesupposita ordinatione rationis ordinantis aliquid in finem.
The will does not ordain, but tends to something according to the order of formal aspect. Consequently this word "intention" indicates an act of the will, presupposing the [actual] ordination of the [formal] aspect that orders something to the end.
Augustinus dicit, in XI de Trin., quod "voluntatis intentio copulat corpus visum visui, et similiter speciem in memoria existentem ad aciem animi interius cogitantis".
Augustine says (De Trin. xi, 4,8,9) that "the intention of the will unites the sight to the object seen; and the images retained in the memory, to the penetrating gaze of the soul's inner thought."