According to the Aristotelian definition, the soul is the first actuality of a natural, organic body that has the potential for life and also contains within it the principle of movement and rest. Actuality is the form of being that exists in a state of potentiality. The soul, therefore, is a substance in the sense of form. The characteristic Aristotelian formulation is that the soul is what a particular body was meant to be. It is the cause and principle of the living body, the source and purpose of its movement. It is the cause as the form of the ensouled body. (…) At the same time, he came to see clearly that the soul and the body constitute, in the case of humans as well, a very cohesive unity. The work "On the Soul" marks the final stage of Aristotle's development in the field of psychology; this is evident from the entire preceding course.