Since sensible substance is changeable, those changes to happen, whether movement or modification, are caused. Potentiality and actuality are now to be considered in these causes and in essence are part of the material, final and formal cause. The final cause is that which urges the change to be made the state of greatest good of a thing. All things must share the greatest good as well as be something to which the final cause aims toward. Aristotle, in his explanation of movement in the eternal, sees this as thought.
Thought is capable of both possession and reception.
The Treatise on the Soul explains the soul in means that relate to metaphysics. It is explained that matter is potentiality and form is actuality and that the body is the matter. The body is the matter because the body has life. Since matter must be capable of existing in many forms, thereby possessing potentiality, the soul must be the substance. Acting as substance the soul is also the actuality of the body.