Amor et passio (proprie quidem, secundum quod est in concupiscibili; communiter autem, et extenso nomine, secundum quod est in voluntate) quia amor consistat in quadam immutatione appetitus ab appetibili.
Love is a passion (properly so called, according as it is in the concupiscible faculty; in a wider and extended sense, according as it is in the will) because love consists in a change wrought in the appetite by the appetible object.
Passio est effectus agentis in patiente. Agens autem naturale duplicem effectum inducit in patiens, nam primo quidem dat formam, secundo autem dat motum consequentem formam.
Passion is the effect of the agent on the patient. Now a natural agent produces a twofold effect on the patient, for in the first place it gives it the form, and secondly it gives it the movement that results from the form.
Sic etiam ipsum appetibile dat appetitui, primo quidem, quandam coaptationem ad ipsum, quae est complacentia appetibilis; ex qua sequitur motus ad appetibile.
In the same way the appetible object gives the appetite, first, a certain adaptation to itself, which consists in complacency in that object; and from this follows movement towards the appetible object.
Prima ergo immutatio appetitus ab appetibili vocatur amor, qui nihil est aliud quam complacentia appetibilis; et ex hac complacentia sequitur motus in appetibile, qui est desiderium; et ultimo quies, quae est gaudium.
The first change wrought in the appetite by the appetible object is called "love," and is nothing else than complacency in that object; and from this complacency results a movement towards that same object, and this movement is "desire"; and lastly, there is rest which is "joy."
Philosophus dicit, in VIII Ethic., quod amor est passio.
The Philosopher says (Ethic. viii, 5) that "love is a passion."
Unio pertinet ad amorem, inquantum per complacentiam appetitus, amans se habet ad id quod amat, sicut ad seipsum, vel ad aliquid sui. Et sic patet quod amor non est ipsa relatio unionis, sed unio est consequens amorem. Unde et Dionysius dicit quod "amor est virtus unitiva", et philosophus dicit, in II Polit., quod unio est opus amoris.
Union belongs to love insofar as by the complacency of the appetite, the lover stands in relation to that which he loves, as though it were himself, or part of himself. Hence it is clear that love is not the very relation of union, but that union is a result of love. Hence, too, Dionysius says that "love is a unitive force" (Div. Nom. iv), and the Philosopher says (Polit. ii, 1) that union is the work of love.