Why Believe in the Best Theory?
This argument is not deductively valid:
The best available theory says p;
Therefore, p.
For even the best available theory can be false. It's not even clear that the premiss makes the conclusion very probable. So is it fallacious to argue for a claim by pointing out that it is entailed by the best available theory? No. The argument may be valid in another sense: in the sense that it is irrational to accept the premiss but reject the conclusion. For if you accept that the best available theory says p, rejecting p means to knowingly reject the best available theory -- and that may well be irrational. It's always irrational to knowingly reject the best available theory in favour of another, worse, theory. The only rational alternative is agnosticism. But if the best theory is sufficiently good and much ahead of its rivals then agnosticism too is irrational. That's because rationality demands that you increase your credence in a proposition in the light of good reasons.
The best theory says p
So, p
Dont people normally argue this way:
(1) The best theory says p
(2) We have warrant to assert those sentences which the best theory says are true
(3) To have warrant to assert p is to to have warrant to assert that p is true
So
(4) We have warrant to assert that p is true
(1)-(4) looks valid.