[anatta]

Your opponent just called on the turn. Its hard to put him on an eight on the river as many would chose to raise the turn. Maybe he reads you as very likely to bet river/possible fold to turn raise. I think once he does just call on turn you are doomed to bet river.

There are many medium pocket pair type hands that he can have and will call with. Not that many eights playable preflop, some but its gotta be suited and connected usually. So I think Bayes favors a bet, but the fact you gotta call the raise maybe not.