What conclusion can be made about the spiritual nature of the human being or the soul in Plato's "Phaedo"?
In the Phaedo, Plato through the mouth of Socrates argues for the idea that human beings possess an immortal and sentient soul. This was by no means universally believed in ancient Greece, and so he has difficulty carrying his point. He argues for the soul on four main grounds: cyclical, recollection, affinity, and true nature (the Form of soul).
The cyclical argument declares that there must be something pre-existing that animates us: "...our souls must exist in the other world, for if not, how could they have been born again?" (Phaedo, 70c-d). Life must come from death, and death from life, cycling like other extremes (hot/cold, dark/light).
The argument from recollection or learning depends on Socrates' assertion that all learning is nothing more than recollection of knowledge that is already present. However, for this recollection to take place, the body must be animated by a soul that carries with it information from past lives.
The third argument relies on the affinity between the fates of the soul and the body. Just as the body continues after death as a corpse, the soul must continue as something, albeit something imperceptible to ordinary human senses.
The fourth argument, the only one considered conclusive, depends on Plato's theory of Forms. A Form exists eternally and unchanging. Therefore, the Form of soul, in essence linked with life, exists in all souls, which are by virtue of this immortal and immutable.