“What is that the inherence of which, will render the body alive? [The soul. ] …Then whatever the soul possesses, to that she comes bearing life? …And is there an opposite to life? [Death. ] Then the soul, as she has been acknowledged, will never receive the opposite of what she brings. …And what do we call the principle which does not admit of death? [The immortal. ] And does the soul admit of death? [No. ] Then the soul is immortal?”
Yes.
Plato, Phaedo
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Socrates 168
classical Greek Athenian philosopher -470–-399 BCRelated quotes

Source: To the Most Reverend Nun Xenia (c. 1344), p. 296

“If the soul does not exist, then stop believing in life and death as well.”
“It is not death; but a bad life, which destroys the soul.”
Sentences of Sextus