Meditations, Book 4, Section 17

Marcus Aurelius — The Emperor

Some people don't believe souls survive after death. They ask: how could the air hold all these souls forever? I ask back: how does the earth keep holding all the buried bodies forever? Here's how it works. Dead bodies change and break down into something else. This makes room for new dead bodies. The same thing happens with souls. After death, souls move into the air. After some time there, they change or blend back into the original rational substance that all souls come from. This makes room for new souls who are just starting to exist without bodies.

Marcus Aurelius — The Emperor Original

If so be that the souls remain after death (say they that will not believe it); how is the air from all eternity able to contain them? How is the earth (say I) ever from that time able to Contain the bodies of them that are buried? For as here the change and resolution of dead bodies into another kind of subsistence (whatsoever it be;) makes place for other dead bodies: so the souls after death transferred into the air, after they have conversed there a while, are either by way of transmutation, or transfusion, or conflagration, received again into that original rational substance, from which all others do proceed: and so give way to those souls, who before coupled and associated unto bodies, now begin to subsist single.