Board Thread:General Discussion/@comment-31283382-20150404011045/@comment-26091912-20150404061725

To say that Aizen has "eternal life" is a misnomer, because it implies disassociation with the cycle of transmigration, therefore he has no beginning. It also implies he has no end, rendering his immortality absolute rather than potential, therefore to kill him is a physical impossibility.

What you could say is: Aizen can live on indefinitely, but it's extremely difficult to kill him using conventional means; assuming the Hogyoku can be destroyed.