One of the most influental and famous doctors of antiquity. He unfortunately made mistakes that actually did set back medicine (but he also made many correct observations that progressed it too) and due to how scholastics works it also cemented those mistakes during middle ages. I would argue though Galen was not responsible for those but the catholic scholastics who supressed medical research (not to mention even Galen himself could not perform autopsies because they were forbidden even in his own time).