Doubt is the beginning of all knowledge, which is why arrogant people often turn out not to be as wise as they think they are. If you are excessively confident in yourself and fail to inquire and investigate the purported veracity behind your beliefs, then you are not going to listen to other people or challenge yourself intellectually, which means you are not going to learn much at all. This is why Socrates was so much wiser than nearly everyone he encountered—not in the sense that he knew a lot, but in that he genuinely recognized his ignorance, even in light of all the knowledge he had already consumed. The lesson here is that the more you learn, the more aware you are of how little you actually know.
Doubt and knowledge are connected inextricably. The very basis of acquiring knowledge and making meaningful progress is only accomplished insofar as one doubts what one holds to be the case. If you refuse to doubt, then how can you identify potential holes in your reasoning, or uncover alternative perspectives that you may have not considered before? Imagine if we refused to doubt science. Perhaps we would still believe in geocentrism or the electric universe theory. Or what about if doubt never arose in philosophy? Maybe we would still believe that an oligarchy is preferable to a democracy. By questioning standard doctrine, along with our beliefs and assumptions, we can avoid making decisions based on erroneous information, bias, and prevailing cultural prejudices. Doubt leads us to think more critically and objectively.
Sometimes, however, it is probably wise to employ doubt judiciously. If all you do is doubt, then what is even the point of pursuing truth? I am not suggesting that you should doubt to the point of believing absolutely nothing. This would be a form of epistemological nihilism or suicide. Your intellectual journey will move you inevitably to accept certain propositions over others, which is what the intellectual will naturally seeks to accomplish. That is the entire point of creatures endowed with reason: To pursue the good and the true. But a healthy dose of doubt keeps us in check. To doubt something genuinely, is to adopt a degree of uncertainty or to call into question a particular belief. Put differently, a deliberate aversion to accepting a thought before entertaining it is most definitely a sign of a potent and healthy intellect. Unless you wish to remain in a dogmatic slumber, you should probably begin to exercise some doubt.
“The man who never alters his opinions is like standing water, and breeds reptiles of the mind.”—William Blake, The Marriage of Heaven and Hell
"only don't know."
This was the repeated phrase of the great Korean Zen Master Seung Sahn, who taught many in the 1970s and 1980s. Zen even speaks of the vital importance of "Great Doubt."
This is not the petty pseudo skepticism of parapsychology deniers and atheists with no understanding of philosophy or theology. It's the doubt that "I" and the "world" exist in the way we take it to be. It is founded on years of intense discipline understanding the way our minds, emotions and instincts work to build up an apparent self and apparent work..
This is often thought to be a purely Buddhist endeavor, but holy indifference in the Christian mystical tradition, along with Meister Eckhart's and Thomas Merton's repeated injunction to let go of the false self and find the Kingdom of Heaven within, shows that this is fully a part of the deepest Christian tradition as well (as it is of all contemplative traditions around the world, including those of indigenous peoples, not just the so called "major world religions").