Zer0Summoner: The ELI5 is that we assume that the soul is the opposite of the body, and since the body is mortal, the soul is not mortal.
As for why he would just assume that the soul is the opposite of the body, I think the relative looseness of his reasoning probably had something to do with his plans for the following day.
Sinistersnare: Hey Im not an ELi5er or anything, but this semester at my school I took a medieval Arabic Philosophy course, and a lot of the philosophers in that setting are really into talking about the soul. Avicenna talked a lot about it, and I was just listening to this great podcast on his opinion of the immortality of the soul.
[It’s not Socrates but it may interest you](https://www.historyofphilosophy.net/avicenna-soul)