Hylomorphism is the theory that all things are a combination of matter and form. Aristotle was one of the first writers to approach the subject of life in a scientific way. Biology was one of his main interests, and there is extensive biological material in his writings. He believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.
According to Aristotle, all things in the material universe have both matter and form. The form of a living thing is its soul. There are three kinds of souls: the vegetative soul of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the ‘animal soul’ which causes animals to move and feel; and the rational soul which is the source of consciousness and reasoning which Aristotle believed is found only in man.
A properly organized body is already alive simply by virtue of its structure. However, the property of life or soul is something in addition to the body’s structure. The analogy of a car can be used to explain this interpretation. A running car is running not only because of its structure but also because of the activity in its engine. Likewise, a living body is alive not only because of its structure but also because of an additional property. The soul is this additional property, which a properly-organized body needs in order to be alive.