Originally posted by AThousandYoung
[b]That is, given that you justifiably believe propositions A, B, and C, you are not entitled to claim that your belief of the proposition (A and B and C) is justified.
I don't agree. That belief is perfectly justified even in the light of the other justified beliefs. [/b]
No, it's not.
Define these propositions:
Proposition A: The ball is not red.
Proposition B: The ball is not blue.
Proposition C: The ball is not green.
Proposition D: The ball is one of red, blue, or green.
Proposition E: A and B and C and D
Do you agree that E is a logical contradiction? I hope you do. It is a contradiction because its truth table would have a value of false in all cases; no case could ever make E true.
Do you agree that belief in E cannot be justified? I hope you do. Since it is a contradiction, it cannot by definition be more likely true than false, and thus its belief cannot be justified.
Now, I'll assume you agree to those two points.
Suppose somebody's beliefs in the disjoint propositions A, B, C, and D are each justified. This person is justified in believing them severally but not jointly; severally by the supposition, but not jointly as proven above, since E is an unjustifiable belief.
Do you follow? If you do, then you should understand the flaw that I have been trying to point out. It is this: given any set of propositions in which you hold justified beliefs under your justification criterion (e.g. the set {A, B, C, D}), you cannot be sure that the conjunction of their underlying propositions (e.g. E) is not a contradiction. But this means that if you do in fact simultaneously believe everything that you believe, then you may be believing in a contradiction, as just illustrated -- and this belief is itself not justified under your criterion, the very criterion that yields the contradiction.