This is what I've always thought and have told students, too: Vincent and
Gaugin were roommates at the time. They were unknowns and although friends,
they did fight quite a bit about things. (Not that unlikely, considering they
were poor and probably worked in cramped quarters.)
One day, they were fighting and Vincent pulled out a knife, Paul Gaugin ran
off and in order to stop the 'voices' that VG thought were in his head,
tellling him to kill his friend, he cut off a part of his ear in an attempt to
stop the voices. This could also corroborate the tinnitus (sp??) theory about
the ringing in his ears.
I haven't heard anything conclusive about him sending it to a prostitute!!