They know that mistakes occur in the mind of all living humans beings. They also know that things are in a state of constant change or flux. Humans can get into a mindset where the only truth is their own and so convince themselves that nothing can dissuade their opinion. In religion there are those who are convinced that the only truth is their truth. It seems so easy to slip into a position of being close minded. Nothing can penetrate that position.
Absolutes do exist, but we may not be able to identify them. God's existence would be one absolute—His constancy and unchanging person (Malachi 3:6). In the words of scientists who claim it is impossible to have absolute certainty lies an absolute. They are absolutely sure that there is no absolute certainty. We humans are limited and chained by the boundaries of our minds (1 Corinthians 8:2). That is an absolute certainty. That we will die is an absolute certainty (Hebrews 9:27). We were created this way so that we would search for God.