I answer that, Likeness is a kind of unity; hence that which is like us, as being one with us, causes pleasure; just at it causes love, as stated above (Q. 27, A. 3).
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic likeness unity
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

“Therefore likeness is not a cause of pleasure.
On the contrary, Likeness is a cause of love, as above stated (Q. 27, A. 3) : and love is the cause of pleasure. Therefore likeness is a cause of pleasure.
I answer that, Likeness is a kind of unity; hence that which is like us, as being one with us, causes pleasure; just at it causes love, as stated above (Q. 27, A. 3). And if that which is like us does not hurt our own good, but increase it, it is pleasurable simply; for instance one man in respect of another, one youth in relation to another. But if it be hurtful to our own good, thus accidentally it causes disgust or sadness, not as being like and one with us, but as hurtful to that which is yet more one with us.
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