Sex is not only the object, but also the cause of the appetite. We not only turn our view to it, when actuated by that appetite; but the reflecting on it suffices to excite the appetite. But as this cause loses its force by too great frequency, ’tis necessary it shou’d be quicken’d by some new impulse; and that impulse we find to arise from the beauty of the person; that is, from a double relation of impressions and ideas.
It is not enough to want to sleep with another person to be in love with them (such desire is neither reliable nor durable). There is a necessary condition to be in love: that of beauty. It is the glue that allows to eonnect the amorous conditions (tenderness and sex):
Kindness or esteem, and the appetite to generation, are too remote to unite easily together. The one is, perhaps, the most refin’d passion of the soul; the other the most gross and vulgar. The love of beauty is plac’d in a just medium betwixt them, and partakes of both their natures: From whence it proceeds, that ’tis so singularly fitted to produce both.
Beauty is therefore the true cause of the amorous passion. What is beautiful please us, a fortiori when in regards to the supreme passion (the amorous passion).
Hume is nowhere specific by what he means by “beauty” (just as he isn’t for love).
It is obvious that he does not stop at cosmetic beauty, and that anyone can imagine many forms of beauty that pleases them. What is beautiful to someone resides in a purely subjective, interior feeling. We find something beautiful or we do not find it beautiful, it is not a rational choice.
Hume therefore invites us to simply remain attentive to our senses, to our feelings, since reason has practically no power there. What form of beauty will awaken the amorous passion, this is particular to each person.
(Hume does say something: wealth, for instance, naturally produces pleasant impressions, which means it may possibly concur to the esteem of the other towards self.)