When you undertake to solve a question in algebra. Once you solve it, you will spend some time checking the steps to make sure that you have not made any mistakes in the solution. Irrespective of how good you are at mathematics, there is always a possibility that you could have made a mistake.
To err is human.
But the very same people, when it comes to their professions would never bother to check the steps that they took to arrive at their conclusion. How many doctors do you think, sit back and analyse if what they are thinking is right or if they have made any mistakes in arriving at their conclusion? How many CEOs go over all the evidence that they have taken into account to arrive at the decision that they have?
We may discuss and offer up evidence, but we rarely double-check ourselves.
Given how often we humans make mistakes, is it right that we are so certain of the decisions that we make?