Descartes' error is something that people used to think was true, but we now know it is wrong. It has to do with how we think about our bodies and minds. Descartes thought that the mind (our thoughts and feelings) was totally separate from the body (our physical bodies). He thought that you could be a person with a body, but didn't need to have a mind. But we now know that the two are connected. Our minds and bodies are connected, and we need both to be a person. So Descartes was wrong!