The physical part of love is that general desire which urges the sexes to union with each other. The moral part is that which determines and fixes this desire exclusively upon one particular object; or at least gives it a greater degree of energy toward the object thus preferred.
 Jean-Jacques Rousseau, Discourse on the Origin of Inequality Among Men (1755). copy citation

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Author Jean-Jacques Rousseau
Source Discourse on the Origin of Inequality Among Men
Topic union sex
Date 1755
Language English
Reference
Note Translated by G. D. H. Cole
Weblink https://en.wikisource.org/wiki/Discourse_on_the_Origin_of_Inequality_Amo...

Context

“But, setting aside the inadequacy of laws to effect this purpose, which is evident from the crimes and disorders to which these passions daily give rise among us, we should do well to inquire if these evils did not spring up with the laws themselves; for in this case, even if the laws were capable of repressing such evils, it is the least that could be expected from them, that they should check a mischief which would not have arisen without them.
Let us begin by distinguishing between the physical and moral ingredients in the feeling of love. The physical part of love is that general desire which urges the sexes to union with each other. The moral part is that which determines and fixes this desire exclusively upon one particular object; or at least gives it a greater degree of energy toward the object thus preferred. It is easy to see that the moral part of love is a factitious feeling, born of social usage, and enhanced by the women with much care and cleverness, to establish their empire, and put in power the sex which ought to obey.” source