Immortality is not ours. Marcus Aurelius 4.21

Marcus Aurelius explains why individual immortality makes no sense to him. His approach depends upon an ancient notion of soul (psyche) that identifies it with breath & movement (especially movement that involves some kind of repetitive order that is regular without being automatic or precisely iterative). He conceives the soul as something material, albeit of finer matter than other things we observe, and argues from the principle that matter is conserved always, that the universe is not without limits. In his view of things, our souls develop from momentary junctures of enduring matter and mind that exist beyond us, as the ocean lies beyond every drop of water on earth. As my body is made of other bodies and will decay to make more bodies after I am gone, so my soul comes from intelligence that predates me, and it will eventually dissolve to become new souls. Body and intelligence appear potentially immortal here, but no individual body or mind is.

Εἰ διαμένουσιν αἱ ψυχαί, πῶς αὐτὰς ἐξ ἀιδίου χωρεῖ ὁ ἀήρ; πῶς δὲ ἡ γῆ χωρεῖ τὰ τῶν ἐκ τοσούτου αἰῶνος θαπτομένων σώματα; ὥσπερ γὰρ ἐνθάδε ἡ τούτων μετὰ ποσήν τινα ἐπιδιαμονὴν μεταβολὴ καὶ διάλυσις χώραν ἄλλοις νεκροῖς ποιεῖ, οὕτως αἱ εἰς τὸν ἀέρα μεθιστάμεναι ψυχαί, ἐπὶ ποσὸν συμμείνασαι, μεταβάλλουσι καὶ χέονται καὶ ἐξάπτονται εἰς τὸν τῶν ὅλων σπερματικὸν λόγον ἀναλαμβανόμεναι καὶ τοῦτον τὸν τρόπον χώραν ταῖς προσσυνοικιζομέναις παρέχουσι. τοῦτο δ’ ἄν τις ἀποκρίναιτο ἐφ’ ὑποθέσει τοῦ τὰς ψυχὰς διαμένειν.

χρὴ δὲ μὴ μόνον ἐνθυμεῖσθαι τὸ πλῆθος τῶν θαπτομένων οὑτωσὶ σωμάτων, ἀλλὰ καὶ τὸ τῶν ἑκάστης ἡμέρας ἐσθιομένων ζῴων ὑφ’ ἡμῶν τε καὶ τῶν ἄλλων ζῴων. ὅσος γὰρ ἀριθμὸς καταναλίσκεται καὶ οὑτωσί πως θάπτεται ἐν τοῖς τῶν τρεφομένων σώμασι, καὶ ὅμως δέχεται ἡ χώρα αὐτὰ διὰ τὰς ἐξαιματώσεις, διὰ τὰς εἰς τὸ ἀερῶδες ἢ πυρῶδες ἀλλοιώσεις.

Τίς ἐπὶ τούτου ἡ ἱστορία τῆς ἀληθείας; διαίρεσις εἰς τὸ ὑλικὸν καὶ εἰς τὸ αἰτιῶδες.

If individual souls persist intact, how then is the atmosphere constantly able to contain them all? And how does the earth make room for all the corpses buried within her for so many ages? Just as evolution and digestion make room in the earth for new corpses, removing the old entirely after a sufficient period of time, so they work upon the breath of life that is each soul. Released from its body, the soul endures for some time, shifting and turning in the air, melting and burning as it taken up into the fertile seed and source of universal intelligence. By this process, old souls pass away, making room for the new that take up residence by them. This is the answer to give when anyone proposes the idea that souls are eternal.

But it is not enough to consider only the multitude of human corpses buried with proper rites: we must also remember how many animal corpses are eaten each day, consumed by us and by other predators. What a vast throng is devoured, buried inside the bodies of those who take nourishment from meat! Nature receives these bodies eventually, too, when she drinks our blood, sucks our air, and snuffs our little fire.

What is the truth we find, when we inquire into the fate of the individual? Dismemberment that removes our little matter and mind into the vast reservoir of material substance and intelligent harmony that constitute the universe.