On Saturday, a man will display precise, rational thinking when fixing his lawnmower. On Sunday, the same man will go to church and fantasize the universe was created in six days.
I believe the key to understanding the difference between the Saturday man and the Sunday man is to first recognize that both are looking for results. Different kinds of results but results nonetheless.
The Saturday man wants his mower to run, while the Sunday man wants emotional assurance of salvation. To get his mower to run, the Saturday man is forced to display precise, rational thinking. But to obtain the result he wants on Sunday, he is forced to cast aside precise, rational thought, and indulge in fantasy. He does this willingly because he wants the emotional result he gets from doing it.
In other words, for most people, rational thinking in itself is not nearly as important to them as whatever results they happen to be looking for. If they are looking for results that happen to be best brought about by rational thought then they will prefer rational thought. But if they are looking for results that happen to be best brought about by fantasy then they will prefer fantastic thinking to rational thought.
It is therefore quite often a mistake to think that a person is either rational or irrational. Most people are both -- switching from one to the other depending on which best brings about the results they want.
By the way, this is one of the more subtle reasons why so many Eastern sages recommend people approach thinking about things without regard for the outcome of their thoughts.