Logical Bias

Here is the logic behind bias. If your argument disproves something I know, this disproves your argument. Proof by contradiction.

Actually, this means that I must choose between my prior belief and your argument, but any number of biases (confirmation bias, familiarity, etc.) will push me to reject your argument.

Sloppy: assume A (my previous belief) and B => -A

A => -B
A & (A => -B)
-B

What that actually proves is (A & (B => -A)) => -B, which says nothing about the truth of A or B.

Better: assume A, B, and B => -A

A => -B
A & (A => -B)
-B
B & -B
-A | -B | -(B => -A)
(B => -A) => (-A | -B)

So, if B undermines A, I have to choose one or the other. Even if you do the logic right, it's tempting to pick -B. Or going back in the sloppy direction, if I am really certain about A, nothing you can say about B, no evidence, no logic, matters. If I "know" A, and you want to convince me B & (B => -A), you're out of luck. In a more ideal world, I should be more curious, but it is what it is.

This is why skeptics try to not "know" anything. Of course, that may lead to other problems.