The first time I can remember hearing of William Harvey was in a history of science class in college. I was blown away to learn that we have only known that blood circulates for 400 years. If you don’t know the history this is a good source but here is the short version:
Ancient people had no idea what blood was doing or what the heart was for but they were able to figure out that arteries and veins are different. Arteries seemed to originate in the heart and veins came from the liver. They also knew that both arteries and veins were filled with blood (but somewhat different blood) and that blood has to be distributed to the whole body through vessels that are too small to see, since if you get a small cut pretty much anywhere it bleeds without any visible blood vessels going to it. They knew that both veins and arteries branched off endlessly into channels too small to see but never considered that they were connected to each other. In the 2nd century AD Galen extended these existing beliefs with more observation and analysis, but didn’t fundamentally challenge them. In his model venous blood originated in the liver and carried nutrition to the body, with the right side of the heart participating in that process. The left side of the heart and arteries and lungs delivered fresh air and cooled the body, and the fact that arteries always seemed to be full of blood rather than air was just kinda brushed off.
Over the next 1000 years apparently very little was done in terms of even dissecting animals or human cadavers, maybe because people figured they knew everything they needed to know, so there was no progress. Between about 1200 and 1600 people started doing observations again and figured out that the sides of the heart weren’t completely separate but that maybe blood was getting from one side to the other across the lungs. And also Da Vinci figured out that the heart was a muscle. Then William Harvey came around and just figured out how things actually worked. He made a lot of observations and did a lot of thinking but the his two strongest points were as follows: First he pointed out that if you take arterial blood and venous blood out and let it sit somewhere then it ends up looking just the same, so its probably the same stuff. Second he pointed out that the speed with which blood seems to move through the body makes it completely implausible that the liver is making it all and then it is just all getting eaten up by the body. Then he said, well, since its the same stuff and it can’t be being created and destroyed in real time, its probably just going around in a closed loop.
There are a decent number of people who seem to believe that Earth is flat. And there are tons and tons of people who don’t believe in evolution, but no one questions the idea that blood circulates. Even though Jesus didn’t know about it. Mohammed didn’t know about it. William freaking Shakespeare didn’t know about it. It is just blatantly obvious! There is a giant pump in the middle of the body with tubes filled with blood going in and out. Of course the blood is circulating. To me the story of William Harvey and circulation, more than any other story in the history of science, is that what is obvious to us was not obvious until some genius put enormous effort into making it obvious. Heliocentrism is not at all obvious. Special relativity is not obvious. Atomic theory and anything related to chemistry are not obvious. Even Newtonian mechanics is not really obvious because most things don’t act like F=ma in our high friction world.
To me the story of circulation is a reminder to not take knowledge for granted. There are most likely obvious facts staring me in the face every day that I can’t see because I am making some dumb assumption that I don’t even know I am making. The hardest intellectual work is often not even to question your assumptions, but to articulate what assumptions you are actually making. Harvey clearly articulated all the assumptions that went into the Galenic model of blood flow and then questioned them and found something that should have been obvious for millennia.