He used a more precise definition of the word "substance" than is currently popular: that a substance is something which can exist without the existence of any other substance. For many philosophers, this word or the phrase "mental substance" has a special meaning.

Descartes sought a foundation of knowledge that could not be doubted. He employed a method of radical doubt, questioning all beliefs that could possibly be uncertain. However, Descartes realized that the very act of doubting presupposed a thinking subject. If he was doubting,