Aristotle believes that all material substances are matter and form. If you remember from the four causes, matter is one cause and form is another cause. Substance theory says that substances are the ultimate things in the universe. Aristotle defends his position on material substances in his book Metaphysics. Matter and form are parts of substances, but they are not parts that you can divide with any technology. Instead matter is formed into a substance by the form it has.
According to Aristotle, matter and form are not material parts of substances. The matter is formed into the substance it is by the form it is. Consider a particular plant. That plant is a material substance. So it has both matter and form. The form is the arrangement, nature and state of the plant. So a plant has an internal order that changes as the plant matures. It gains nutrients from the environment, removes waste and grows. Each of these functions is a part of the plant. It is not possible to separate the activity of the plant from the actual plant. This is even true at the cellular and atomic level. The cells of the plant are plant cells doing the activities needed to happen in that particular plant. The atoms are joined in a pattern that has different properties than any of the individual atoms, but that serves the needs of the plant. At all levels, the plant’s form determines the properties and activities of every physical part of the plant. The matter just is determined by the plant so that particular activities and properties appear. This is Aristotle’s understanding of matter and form.
Aristotle believes that this applies to every material substance – not just plants. He also believes that without a form, matter would have no properties or activities at all. This is the prime matter that Aristotle does not believe in. Since something cannot exist at all if it has no properties or activities at all, prime matter does not exist. Therefore, every material thing has a form. Since matter and form combine to make material substances, all matter is formed by a form to make a material substance.
Aristotle believes in this particular account of matter and form because it uses substance theory and the four causes to explain how to connect form and matter. Logically speaking, it is the only way to connect the two theories. Form explains what makes substances one thing rather than many things. So matter and form cannot be two different (material) things. They must join together to make one thing – the same thing that the form is. Therefore, this entire account depends on the accuracy of substance theory and the four causes.