Rationality is the habit of acting by reason, which means in accordance with the facts of reality. The only alternative is acting by whim, which because reality is absolute, will result in undesired consequences. This is because an action based on a belief in a particular cause-effect relationship will not occur if that relationship is invalid.
Rationality is in your self-interest because the only way to achieve desired outcomes is to act according to reality. To understand reality, one must use reason consistently. Any deviation can have long term problems since one's knowledge is often derived from one's previous knowledge. To accept a false belief once can have the effect of polluting all further knowledge, until the mistakes are cleared away and the new knowledge re-evaluated.
Rationality does not mean being a perfectionist in one's thoughts and ideas. It does not require you to spend enormous amounts of time evaluating every idea. It does not require you to learn everything there is to know, to become an expert at every topic. Rationality means acting according to reason. It means accepting only that which you have reason to believe. It means using logic to weed out any contradictions. It means when you have to accept the judgment of another, you use your own mind to determine whether you should. Is the person educated in that field? Is it knowledge that someone is capable of having? From what you know about the rest of his ideas, is he someone you believe will be correct? Rationality is foremost a method of survival. It is a virtue only to the extent that it encourages one's survival.
- Rationality, Importance of Philosophy as in June 2018