Hume, D. (1789). Of the amorous passion, or love betwixt the sexes (L. A. Selby-Bigge, Ed.). In D. Hume & L. A. Selby-Bigge (Ed.), A treatise of human nature (pp. 394-396). Oxford, England: The Clarendon Press.
http://dx.doi.org/10.1037/12868-059
Of all the compound passions, which proceed from a mixture of love and hatred with other affections, no one better deserves our attention, than that love, which arises betwixt the sexes, as well on account of its force and violence, as those curious principles of philosophy, for which it affords us an uncontestable argument. 'Tis plain, that this affection, in its most natural state, is deriv'd from the conjunction of three different impressions or passions, viz. The pleasing sensation arising from beauty; the bodily appetite for generation; and a generous kindness or good-will. The origin of kindness from beauty may be explain'd from the foregoing reasoning. The question is how the bodily appetite is excited by it. (PsycINFO Database Record (c) 2016 APA, all rights reserved)