Rational thinking is performed on a platform of solid evidence. The beliefs we form from rational thinking are reevaluated when new evidence emerge that prove a current belief wrong.
And so, rational thinking is the search for the objective truth, and we form beliefs based on our current knowledge and iterate these beliefs when we acquire knowledge that prove them wrong.
The exception to this is confirmation bias.
An example of confirmation bias is when someone have a pre-decided opinion about something. So for example if you decide that your opinion is that everyone who lives in a certain area are mean, you are now going to acknowledge all evidence of this being true and ignore all evidence of this being wrong, thus "making" your assumption right.
This is why the importance of keeping an open mind is extremely important in general in life, otherwise you can slip into confirmation bias unwillingly.