According to the Aristotelian definition, the soul is the first actuality of a natural, organic body that has the capacity for life and also contains the principle of motion and rest within it. Actuality is the form of a 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 specific body was meant to be. It is the cause and principle of the living body, the source and purpose of its motion. It is the cause as the form of the animate body. At the same time, he came to see clearly that the soul and body constitute, in the case of humans as well, a very cohesive unity. "On the Soul" marks the final stage of Aristotle's evolution in the field of psychology; this becomes evident from the entire course that preceded it.