Not all old medicine was not effective. I think your question ignores a *lot* of valid, if primitive, medical practices that have value.
But anyways, because they didn’t have a lot of options for huge double-blind scientific studies. They gave someone some herbs, and sometimes the person lived. Therefore the herbs healed them. Obviously, this doesn’t really work because sometimes people who seem very sick get better for whatever random reason. Or maybe the herbs really did help. It’s impossible to say without a well-designed scientific study.
Latest Answers