This simple point shouldn't need an explanation, but it seems it does.
Far too many times I've heard people state that their mind is made up, and nothing is going to change it. If you've reached the point where nothing can change your mind, not even reasoned argument based on solid facts, then it's a pretty fair bet that you're not being reasonable.
It is a fundamental bedrock of science; That any knowledge is open to re-evaluation in the light of new evidence. It's what makes science differ from religion as a belief structure; Scientists are willing to question their own 'beliefs'. Scientific 'beliefs' change as the evidence does.
By definition, at the point where you are not willing to question new evidence, because the old dogma suits you fine, you're not using your ability to reason. Certainty is the death of reason.
It's an argument we have to be careful with. There are levels in all things, and obviously we can be 'more certain' (if that's not a tautology) of some things than others. We simply have to be aware of the dangers of being absolutely certain of our views, in a world where new evidence comes to light daily on a multitude of subjects.
So we can't be certain. But this doesn't mean we can't seek to increase our understanding. It doesn't mean that ignorance is as valid as knowledge. It simply means that however much we know, we should always be willing to change our views in the light of better evidence, and sound reason.