Rhazes was a man a long time ago who was very, very smart. He lived in a place called Persia and he wanted to help people who were sick or hurt.
So Rhazes did lots and lots of studying and learning about the human body and how it worked. He wanted to figure out why people got sick and what could make them better.
After all his studying, Rhazes became a great doctor. He would help people by giving them special medicines and telling them what they needed to do to feel better.
Lots of people liked Rhazes and wanted him to teach others how to be doctors like him. So he wrote lots of books to help them. His books were very popular and were used to teach medicine to other people for a really long time.
Even though Rhazes lived a long time ago, his work still helps doctors today!