Socrates was a Greek philosopher, who lived in Athens from 470 BCE to 399 BCE. He did not leave behind anything written, but he and his talks appear in the works of his disciples Platon and Xenophon.
As for the sovereign part of the human soul, we should consider that God gave it to be the Divinity in each one, it being that which, inasmuch as we are a plant not of an earthly but a heavenly growth, raises us from earth to our brethren in heaven. When one is always occupied with the cravings of desire and ambition which he is eagerly striving to satisfy, all his thoughts must be mortal, and, as far as it is possible to become such, he must be mortal every whit, because he has made great his mortal part. But he who has been earnest in the love of knowledge and true wisdom, and has exercised his intellect more than any other part, must have thoughts immortal and divine. If he attains Truth, in so far as human nature is capable of sharing in immortality, he must altogether be immortal. And since he is ever cherishing the divine power, and has duly honored the Divinity within, he will be supremely happy. Plato, "Timaeus"
The true lover of knowledge is always striving after Being—that is his nature; he will not rest at those multitudinous particular phenomena whose existence is in appearance only, but will go on—the keen edge will not be blunted, nor the force of his passion abate until he have attained the knowledge of the true nature of all essence by a sympathetic and kindred power in the soul. And by that power, drawing near and becoming one with very Being, ... he will know and truly live and increase. Then, and only then, will he cease from his travail. Plato, "Republic"
The immortality of the soul is demonstrated by many proofs; but to see it as it really is—not as we now behold it, marred by communion with the body and other miseries—you must contemplate it with the eye of reason in its original purity; and then its beauty will be revealed. Plato, "Republic"
When a person starts on the discovery of the Absolute by the light of the reason only, without the assistance of the senses, and never desists until by pure intelligence he arrives at the perception of the absolute Good, he at last finds himself at the end of the intellectual world... Plato, "Republic"
The Death of Socrates by Jacques-Louis David (1748-1825)
Accused by the Athenian government of denying the gods and corrupting the youth through his teachings, Socrates was offered the choice of renouncing his beliefs or being sentenced to death by drinking hemlock. Against the request of his followers he chose death. The French artist David shows him calmly discoursing on the immortality of the soul with his desperate disciples, while he is accepting the goblet with the poisonous liquid.