Galen of Pergamon — the original Doogie Howser, M.D. — became a doctor at 16 and educated himself by reading the texts of Hippocrates, the father of Western medicine who preceded Galen by about 500 years. He subscribed to the Hippocratic notion of the four humors — blood, phlegm, black bile and yellow bile — imbalances of which were thought to cause various emotional or psychiatric conditions. Previously, such emotional states and mental afflictions were considered supernatural occurrences and the direct interference of the gods.
Galen believed that love was the result of all four fluids mixing together at once. Based on observed symptoms like sleeplessness, quickened pulse and moodiness, Galen saw love as an affliction. The view that observable love was a medical illness remained until the 1600s, when the Hippocratic principle of humors also fell out of favor.
Love isn’t just fodder for poets. For as long as people have been falling for each other, scientists have worked hard to locate the roots of love somewhere in the body. The ancient Greeks and medieval men of medicine believed that imbalances in bodily fluids like phlegm and blood were responsible for that weak-kneed, goofy-smiled condition of longing, but as early as the 1660s researchers had begun to grasp at the brain’s role in romantic love. Modern-day scientists know a lot more about how the emotion works — it involves the brain’s reward centers and pleasure chemicals like dopamine — but it’s still up for debate whether science or poetry describes it better.