Ouroboros said:
4) I don't believe proposition X is false
Don't believe X is false =/= believe X is true.
Don't believe X is false can be because the person got no convincing evidence to prove X is false.
But for the person to believe X is true, for him it got to have convincing evidence to prove X is true first.
Without evidence to prove X is false =/= with evidence to prove X is true.
Without evidence to prove X is false =/= X have already been proven true.
Without evidence to prove X is false can be the possible situation that the evidence is pending to be found.
The evidence to prove X is false, is pending to be found =/= X have already been proven true.
In order to say X is true it got to have convincing evidence to prove it's actually true first.
Also i'm not sure if this question is related to the things i've been said above, this question ---> "can we prove the existence of negative/non-existence doesn't exist in all situation?" .
Without evidence to prove negative/non-existence doesn't exist =/= negative/non-existence have already been proven exist.