The Timaeus gives us a third take on souls from Plato’s perspective (after Phaedo and Republic). This dialogue links the other two together in giving us a clearer picture of what Plato considers divine and eternal, but also a stronger idea of the non-eternal aspects of the soul. The text attempts to set forth the causes of the world as we know it.
Plato speaks of an artisan crafting the universe. He differentiates between the Artisan and the gods of the heavens (Zeus, etc.) so as to reconcile the Greek mythology with a more centralized philosophical picture. The artisan makes the universe and souls, but leaves it to the gods to fill out the physical accompaniments. This two part creation (first souls, then bodies for them) will be picked up by Augustine and later Aquinas. For Plato it works to explain why our souls are perfect, while our bodies are susceptible to change and decay.
The artisan first forms the soul and intelligence of the whole cosmos, which Plato compares to one perfect animal. It must be ensouled because it moves and has order in it. The cosmic soul has to do with ratios between perfect numbers manifest in perfect spheres. The physical cosmos is then shaped out of matter and aligned with the cosmic soul, so that the planets and stars mark out time in the ratios of the soul.
From the leftovers (and in the same container), the artisan makes souls for four species within the cosmos: gods in the heavens, birds in the air, beasts on earth, and sea creatures in the waters. First came the gods (corresponding to stars and planets). These have perfect purpose, being able to move only in circles, ever forward on their paths or revolving about themselves. The souls of men come next, and the gods fashioned them into physical bodies that confused the perfect motion of their souls, so that they are capable of moving in many directions, and out of their true course.
Plato speaks explicitly here of irascible and appetitive souls. The divine soul, made by the artisan, gives us the power of intellect. In trying to fit it to a body, the gods, somewhat poorly attempted to copy the rational soul to make an organizing principle for the bodies. Those copies order and move the body, to which the divine soul is attached. Because the divine soul is more perfect, it longs to be free. Because the physical souls are not, they tempt the divine soul.
Thus the gods create to divide the eternal from the mortal souls. Plato says the intellectual soul resides in the head and must regulate the passage of energy between itself and body. The irascible soul (source of anger, pride, passion…) resides in the heart. It can lure the intellect away from virtue, but it can also help the intellect in ruling the appetites. The appetitive soul (source of bodily desires like hunger) resides in the liver. The diaphragm acts to insulate the heart, just as the neck defends the head. The three souls match exactly with the three types of soul in the Republic.
Things get more interesting when Plato speaks of our relationship to the souls of other creatures. He maintains that plants have desires as we do. They eat and drink and reproduce, so they must have appetitive souls, but he does not go into details about where they come from. The souls of animals, on the other hand, come from men.
Plato maintains that virtuous men, with their thoughts on heaven and a love of wisdom (philosophia) achieve escape in death. Their intellectual souls flee their bodies to reside in heaven with their corresponding star. (The number of stars and souls is equal.) Less virtuous men are reincarnated in a lower state as s result of their poor focus. First they are reincarnated as women. If they still fail in their attention, they come back as other animals. If their thoughts are on heaven, but through the senses and not the intellect, they come back as birds.It they value the passions of the heart over the intellect, they come back as land animals. If they transgress, if they value bodily desires above all, they come back as sea creatures. Note that Plato’s word for the irascible soul can be translated as spirited, enthused, or inspired. The sea creatures come from those who do not value their breath properly. Through a long series of generations, these souls may be redeemed, brought back to humanity and, eventually, freedom. The notion is not unlike forms of Buddhism.
Here, for the first time, we see a system with clearly divided eternal and mortal souls, a concept that will recur throughout Western thought for two millennia, before we lose sight of the animal and vegetable souls all together. In the meantime, Aristotle will be even more precise about them and speculate more openly and positively on their relationship with the intellectual soul of humans.