Descartes begins with this essential idea: A substance does not rely on any other thing for existence. It is completely independent.
Ultimately, he later clarifies, God is the only thing that depends on nothing else for survival.
Descartes then claims that there are other things in the natural world created by God which exist independently and without a dependence on any other thing except God. These things are called created substances. A rock, for example, is a substance. God created the rock, and it relies on nothing else in order to exist as a rock.
A substance has one attribute, which is the one property that is foundational to the substance. This makes all substances one of two types: extended substances—meaning that the substance has length, depth, or breadth—or a thinking substance—meaning the mind.
All substances are therefore either body substances, meaning that they have shape and/or have a volume, or mind substances. Because Descartes believed in only these two types of substances, he became known as a substance dualist.
Descartes is known for his claim that human minds and bodies are separate substances and exist as fully complete substances independent of each other. All consciousnesses are substances. All bodies (whether a rock, flower, dog, or human body) are also substances. There has been some debate about how Descartes's ideas regarding embodied humans, consisting of both a physical body and a conscious mind, can exist comprised of two separate substances. Descartes referred to this principle as a formed unit and allowed that humans are "exceptional beings."