Descartes and Substance Dualism
Descartes believed that mind and body are wholly different substances that don't depend on each other to exist. This idea completely separates your physical body from your thinking mind.
His famous argument uses divisibility to prove this separation. Your body can be divided into parts - you can lose a leg or an arm and still be you. However, your mind can't be chopped up in the same way thoughcriticspointtomultiplepersonalitydisorderasapotentialcounter−example.
Descartes also used doubt as a key argument. You can doubt almost anything about your physical existence, but you can't doubt that you're thinking. This led to his famous phrase "Cogito, ergo sum" - "I think, therefore I am."
Key Point: Since you can doubt your body exists but not that you're thinking, mind and body must be separate substances.
The core idea is that thinking takes place in the mind as your essence, proving you must exist as a thinking being. Your senses can deceive you, and it's sometimes hard to tell dreams from reality, but the act of thinking itself can't be doubted.