My dissertation investigates the origin and development of philosophical argumentation about the immortality of the soul in the ancient Platonist philosophical tradition. I argue that Plato’s theory is best understood not as requiring the actual everlasting existence of the soul in an afterlife, but as the view that the human soul is akin to the divine due to its privileged cognitive access to the intelligible world. I further show how Plato’s theory was creatively reinterpreted or misunderstood by later thinkers like Plotinus and Augustine, who then provided the conduit through which Plato exercised a lasting influence on Western thought.