I am reading Bill Bryson's At Home: A Short History of Private Life, which is sparking much thought, sending me off in new directions. His framework is the Norfolk rectory he has moved into - he has apparently moved back to the UK, taking each room in turn to trace history. The kitchen, for example, leads to long discussions of food, agriculture, servants, hygiene, and safety; the fuse box to discussions of fuel and labor-saving; the dining room to class discussions and the spice trade. Quite fascinating, especially just after reading McWilliams's A Revolution In Eating, which covers overlapping material far less interestingly.
I am at the discussion of spices, nutrition, and vitamins, impressed by how late any accurate information has come to mankind on the topic of what foods are good for us. Even with a few centuries of practical evidence that something in citrus fruits prevented scurvy, that evidence mostly remained restricted to navies, and others who were away from fresh plant foods of any kind for long periods. Many leading nutritionists remained unconvinced that it was a deficiency disease, well into the 20th C. They had other theories, such as constipation, bad air, weak constitution, and the like. Food was just something you ate, living if you got some of any kind, dying if you didn't.
I contrast this knowledge, that different foods have different necessary things in them, now known at least vaguely to even young children, to far more complicated ideas that were discovered earlier. Einstein's Theory of Special Relativity predates knowledge of a balanced diet.
I have two related speculations why this would be. Evidence about nutrition and survival is necessarily ambiguous until one does rigorous experiment. Even now, people get sick and we wonder why or what it is; we speculate whether it is viral, bacterial, attributable to emotional cause, something we ate, or a host of other possibilities. This tribe eats food x and they mostly live but some of them die, while that tribe eats very different food y and most of them live while some of them die.
We form theories before the data is in, which is probably a useful trait, which kept us alive these many centuries. Maybe it's the water at that stream we stopped at. It must be poison/evil spirits/wrong time of year. Don't drink it. The theory, even if inaccurate, embeds the lesson.
Yet once one has a theory, as we know, it is tough to dislodge. I know a science teacher who is only partially moved by the knowledge that the entire autism/vaccination connection is founded on fraud. He still thinks that because the link has not been definitively disproven to his satisfaction that it still bears watching. He wants to hold his antivaccination theory. The data doesn't convince him.
Next up: slaves and servants.