- (Premise) Many people know that God exists.
- (Premise) If p is known, then p is true.
- Therefore, God exists.
On the other hand, if someone claimed that platypuses are reptiles, it would not be out of place to persuade her by telling her that biologists know that they are mammals. Yet if the move from (1) to (3) is question-begging, surely the move from:
- Biologists know that platypuses are mammals
- Platypuses are mammals
One option is to take the "know" in (4) as "claim to know". In that case, the argument from (4) to (5) is a non-deductive argument using the suppressed premises
- Biologists are the relevant scientists to ask about platypuses' mammalian status
- What the relevant scientists all claim to know is likely true.
- What many people claim to know is likely true.
Could there be a person who is rational in accepting (3) on the grounds of (1)? I think so. A person might rationally believe that there are persons of such an intellectual carefulness and honesty that when they claim to know, it is very likely that they do in fact know. One might then come to believe that there are many theists who claim to know theism to be true and who fall in this category. Thus, likely, (1) is true. And hence so is (3).
It's hard to come up with a sound argument that couldn't be rationally helpful, unless the conclusion is literally one of the premises.