Atoms were something eternal that could not be observed, but they formed the basis of the observable world. Similarly, for Plato the ideas were something unchangeable and eternal, but they formed the basis of the observable and changing world. Thus Plato shared the basic idea of Democrit of the way of explaining the observable world with something which could not be observed, or the change with something that could not change. As the identity of man was something unchangeable it had to be based on the world of unchangeable ideas. Thus, according to Plato, the soul was part of this world.
Soul is the basic principle of life and movement, and therefore all living creatures have a soul.
Here Aristotle laid down the principles, which are even now relevant for the developmental psychology. He also proposed the basic hierarchical scheme of psychological analysis from simple forms into the complex ones, from sensation to perception, imagination, to general concepts and categories. The controversy between Plato and Aristotle was continued in the Middle Ages by St. Augustin and Thomas Aquinas, respectively. The truth radiates from the God into the soul of man, not through senses, but when man is co-operating with the God. Because the soul is in the inner world and body in the outer there must be some connection between them which is accomplished by breathing.