The green salad, tossed with oil, vinegar, and salt, has been on the human table at least since the days of the Greeks and Romans, and—just as we still bicker over when it’s best to eat it, before the main meal or after—the ancients couldn’t make up their minds either.

Physicians Hippocrates and Galen, on the grounds that salad enhanced digestion, pitched for salad first; a host of gourmets, on the grounds that vinegar dressings made wine taste awful, insisted on salad last.

Others argued that salad should be eaten just before going to bed, since lettuce, traditionally, was touted as a soporific. Its putative drowsy-making properties come from the distinctive milky juice that can be seen oozing from the ends of