The opposite of intellectual humility is intellectual arrogance, a lack of consciousness of the limits of one's knowledge, with little or no insight into self-deception or the limitations of one's point of view. Intellectually arrogant people often fall prey to their own bias and prejudice, and frequently claim to know more than they actually know.
When we think of intellectual arrogance, we are not necessarily implying a person who is outwardly smug, haughty, insolent, or pompous. Outwardly, the person may appear humble. For example, a person who uncritically believes in a cult leader may be outwardly self-effacing ("I am nothing. You are everything"), but intellectually he or she is making a sweeping generalization that is not well founded, and has complete faith in that generalization.
Unfortunately, in human life people of the full range of personality types are capable of believing they know what they don't know. Our own false beliefs, misconceptions, prejudices, illusions, myths, propaganda, and ignorance appear to us as the plain, unvarnished truth. What is more, when challenged, we often resist admitting that our thinking is "defective." We then are intellectually arrogant, even though we might feel humble. Rather than recognizing the limits of our knowledge, we ignore and obscure those limits. From such arrogance, much suffering and waste result.
It is not uncommon for the police, for example, to assume a man is guilty of a crime because of his appearance, because he is black for example, or because he wears an earring, or because he has a disheveled and unkempt look about him. Owing to the prejudices driving their thinking, the police are often incapable of intellectual humility. In a similar way, prosecutors have been known to withhold exculpatory evidence against a defendant in order to "prove" their case. Intellectually righteous in their views, they feel confident that the defendant is guilty. Why, therefore, shouldn't they suppress evidence that will help this "guilty" person go free?
Intellectual arrogance is incompatible with fair-mindedness because we cannot judge fairly when we are in a state of ignorance about the object of our judgment. If we are ignorant about a religion (say, Buddhism), we cannot be fair in judging it. And if we have misconceptions, prejudices, or illusions about it, we will distort it (unfairly) in our judgment. We will misrepresent it and make it appear to be other than it is. Our false knowledge, misconceptions, prejudices, and illusions stand in the way of the possibility of our being fair. Or if we are intellectually arrogant, we will be inclined to judge too quickly and be overly confident in our judgment. Clearly, these tendencies are incompatible with being fair (to that which we are judging).
Why is intellectual humility essential to higher-level thinking? In addition to helping us become fair-minded thinkers, knowledge of our ignorance can improve our thinking in a variety of ways. It can enable us to recognize the prejudices, false beliefs, and habits of mind that lead to flawed learning. Consider, for example, our tendency to accept superficial learning. Much human learning is superficial. We learn a little and think we know a lot. We get limited information and generalize hastily from it. We confuse cutesy phrases with deep insights. We uncritically accept much that we hear and read—especially when what we hear or read agrees with our intensely held beliefs or the beliefs of groups to which we belong.