Secondly, a thing causes love, as being a way to acquire love. It is in this way that seeing is the cause of loving, not as though a thing were lovable according as it is visible, but because by seeing a thing we are led to love it.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic love cause
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/18755/pg18755-images.html

Context

“Therefore God ought to be loved chiefly and before all out of charity: for He is loved as the cause of happiness, whereas our neighbor is loved as receiving together with us a share of happiness from Him.
Reply Obj. 1: A thing is a cause of love in two ways: first, as being the reason for loving. In this way good is the cause of love, since each thing is loved according to its measure of goodness. Secondly, a thing causes love, as being a way to acquire love. It is in this way that seeing is the cause of loving, not as though a thing were lovable according as it is visible, but because by seeing a thing we are led to love it. Hence it does not follow that what is more visible is more lovable, but that as an object of love we meet with it before others: and that is the sense of the Apostle's argument. For, since our neighbor is more visible to us, he is the first lovable object we meet with, because "the soul learns, from those things it knows, to love what it knows not," as Gregory says in a homily (In Evang. xi) .” source