'S or S'

I've tried to work this one out for myself, but the info is conflicting - possibly a transatlantic thing.
Basically, I'm writing an honours thesis on a bloke called Levinas. So, in my text there's alot of reference to "Levinas's philosophy/theory/whatever".
I've put off sorting this out till now, but until this point I always felt Levinas' was more correct than Levinas's.
Which is right in this context? Or are both acceptable? I think Levinas's actually seems phonetically more accurate, if that makes a