Reading the writings of ancient philosophers requires close attention and careful analysis of concepts, as it is well established that they often speak by analogy.
Let’s start from scratch.
The etymology of the term “metempsychosis” derives from the Greek meta- (transfer), en (within), and psyche (soul); we can translate it as “the passage of souls.”
According to Pythagoras, the soul is confined within a body in order to purify itself; in fact, he asserts that it is only when the soul frees itself from the body that it can perceive and live in a higher world (that of ideas, to quote Plato).
Philosophers debate extensively about the soul’s ignorance; therefore, they often state in their writings that for a soul to purify itself, it must know, grow, and expand.
Pythagoras further asserts that this process of the transmigration of souls applies not only to humans but also to animals and plants, which follow their own cycle.
“Do not ruin or harm a domestic plant, nor even an animal that does no harm to human beings” (“Lives and Doctrines of the Most Famous Philosophers,” VIII, 23).
To break free from this chain, one must return to the state of original purity by devoting oneself to the selfless contemplation of truth and practicing esoteric rituals of initiation and purification (catharsis).
As for Plato, he draws upon the writings discovered by Pythagoras, including the myth of Er, a soldier who died in battle and was resurrected to recount what he saw in the afterlife. (Book X, The
Republic).
In the "Phaedo", Plato argues that the soul survives the body; if it has lived wisely, it will ascend to the divine realm, otherwise it will remain bound to the material world.
In the myth of the winged chariot in the "Phaedrus", he also describes the cycle of reincarnation linked to the soul’s ability to contemplate the hyperuranion.
Finally, to highlight the concept of mind and psyche that transcends the animal spirit, and thus confirm that it is impossible for a human to be reincarnated as an animal, I will cite Aristotle’s concept regarding the classification of the soul.
The philosopher divides souls into:
the vegetative soul, which is found in humans, animals, and plants, and which governs fundamental vital functions: nutrition, growth, and reproduction.
Sensitive soul (animals and humans): This encompasses sensation, perception, imagination, and local movement. It allows animals to react to their environment.
Intellectual soul (humans only): The higher faculty, capable of thought, conceptual knowledge, and rationality. Aristotle considers it partially separable from the body. (De Anima).
Obviously, these are broad concepts, but I have tried to summarize them as best I could.
Humans are destined for the evolution of the soul and the psyche.
Humans, animals, and plants exist at different levels of the soul, since wisdom and the higher psyche are unique to humans.