> So it is not rational to take advantage of the
> irrationality of others?
Egoism would say yes. However, that is another way of saying that egoism
concedes that it is not rational for everyone to accept egoism, which
would mean that egoism is wrong by the generalization principle. To
accept egoism at that point would be irrational. Same goes for relying on
the irrationality of others.
> What if the right theory is not the most rational
> choice?
If you believe that, then the argument is over. There's nothing I can do
or say which could prove that rationality is right, because I must
necessarily do so within the boundaries of rationality. Meanwhile, I
personally am not very likely to be persuaded to accept an irrational
ethical theory, particularly since I *have* a rational ethical theory
which seems to work.
We cannot debate the merits of utilitarianism against the merits of an
ethical system based on close scrutiny of biblical scriptures, because the
Bible is not derived from well-understood, undeniable axioms. Though
attempts have been made to prove that God must exist, I have not seen any
which succeed; if God is deniable then the Bible itself is unfounded.
People believe it anyway, however, because they have faith that it is
true. How can an argument from rationality argue with that?