The most esteemed thinkers of ancient Greece and Rome had sex on the brain -- a lot. And it isn't any wonder, really, considering that the human instinct to mate is as potent a drive as hunger pangs in the belly or a parched tongue in need of liquid relief. Moreover, back then in the BCs, the birds and the bees know-how that's taken for granted today remained shrouded in myth and mystery. But that doesn't mean men didn't take a stab at sussing out what all of the sexy fuss was about.
Fourth century B.C. thinker Aristotle, for instance, believed that the degree of heat a man produces during sex determines a baby's sex. If he's warmer than the woman, a boy it shall be; vice versa gets a girl. Greek contemporary and physician Hippocrates attributed female patients' mental health instability and anxiety to hysteria, or a wandering womb [source: Catonné]. And in "Natural History," written in the 1st century A.D., Roman writer Pliny warned that intercourse would taint lactating women's breast milk.
As science advanced from philosophy to empirical knowledge over time, groundbreaking discoveries throughout the centuries -- like the five on this list -- revealed the reproductive equipment inside the human body and even where monogamous love between two sexually active people comes from.