But the idea of Leonardo drawing in a vasa menstrualis that he couldn’t even see, simply because he steadfastly believed it should be there, as did everyone else at the time, is the sort of thing that keeps me up at night. You see, the ideas that human beings have about reality—what it’s made of, how it works, how we all fit into grander schemata—can change fundamentally. Sometimes, those changes are so dramatic and so far-reaching that it becomes nearly impossible to understand the world the way we did before.
