For when we love a thing, by desiring it, we apprehend it as belonging to our well-being. In like manner when a man loves another with the love of friendship, he wills good to him, just as he wills good to himself: wherefore he apprehends him as his other self, in so far, to wit, as he wills good to him as to himself. Hence a friend is called a man's "other self" (Ethic.
 Thomas Aquinas, Summa Theologica (1274). copy citation

add
Author Thomas Aquinas
Source Summa Theologica
Topic friendship love
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“since movement of the appetite follows apprehension. Now love being twofold, viz. love of concupiscence and love of friendship; each of these arises from a kind of apprehension of the oneness of the thing loved with the lover. For when we love a thing, by desiring it, we apprehend it as belonging to our well-being. In like manner when a man loves another with the love of friendship, he wills good to him, just as he wills good to himself: wherefore he apprehends him as his other self, in so far, to wit, as he wills good to him as to himself. Hence a friend is called a man's "other self" (Ethic. ix, 4) , and Augustine says (Confess. iv, 6) , "Well did one say to his friend: Thou half of my soul." The first of these unions is caused effectively by love; because love moves man to desire and seek the presence of the beloved, as of something suitable and belonging to him.” source