Much of the harm in the world is done by people who think they are doing good.
But suppose we think we know something but we are wrong? How often do we believe things that aren't true? While we are frequently concerned about what we don't know, we may forget to be concerned about things we think we know that are actually false. How serious can it be when people believe things that aren't really true?
Consider World War II. The Nazis believed they belonged to a superior race that was entitled to rule the world. They were wrong. If they had not had this wrong belief, they would not have caused the deaths of huge numbers of people, both in battle and in death camps. Of course many Germans opposed them and no doubt some who supported the Nazis were simply malicious individuals. But the Nazis could not have risen to power without the support of many "good" people - people raising families, people working for a living and obeying the law, but people who were wrongly persuaded that the Nazi cause was a good one. One of the ugliest periods of human history would simply not have happened if many well-meaning people hadn't been deceived. People believing something that wasn't true had a terrible price.
The Japanese in World War II also believed they had the right to rule over their neighbors. They were wrong. They were good, hard working people with a high sense of honor and family loyalty and a belief that was false. As a result there was massive loss of life in the war in the Pacific and Asia.
In Salem, Massachusetts in 1692 women were executed for witchcraft. The good, honest people of Salem were trying to get rid of an evil they believed was destructive to their community. While their actions were tragically wrong, they truly believed at the time that they were right.
On September 11, 2001, 19 Islamic extremists hijacked four airliners and used them to destroy the World Trade Center in New York City and part of the Pentagon, killing about 3000 people. These hijackers were so sure that this action was approved by God that they were willing to face certain death. I don't think any real god approved of this action. It seems clear that these men believed something false and killed many people (including themselves) because of that belief.
We modern Americans don't have this problem, do we? Surely we don't believe false things, at least not anything important. Of course we do. Every political controversy in which there are sincere believers on both sides represents such a case. Every one of us is bound to be wrong in at least some of these cases.
We have wars. People on both sides think they are right. Obviously, at least one side is wrong. Not many people are willing to risk their lives fighting a war if they don't thing they are accomplishing something worthwhile. Normally leaders on both sides have to convince the soldiers and populace that their side is good and the enemy is evil in order to get them to fight. Even the people on the wrong side are usually so utterly and totally convinced of their rightness that they are willing to die and kill others for that belief. Avoiding such a war does not require knowing the actual truth. Simply recognizing the possibility of being wrong could prevent many people from taking such extreme measures.
A common problem that leads to war and other expressions of hatred and persecution is differences in religious beliefs. There are many religions, and most have strict beliefs that are in direct conflict with the beliefs of other religions. Any two religions that are in conflict cannot both be right. Yet all too often the adherents of some religious belief are so convinced of their correctness that they feel justified in killing and persecuting those of other faiths. Even if one of these groups is actually correct, that still means the vast majority of people fighting in religious conflicts were totally convinced they were right when they actually were wrong. Imagine how much human suffering could have been avoided if people simply had the capacity to recognize when they were not sure of particular beliefs!
While wars and persecution are some of the worst consequences of false beliefs, there are many other ways in which believing what is not true can hurt us.
Christian Scientists firmly believe that it is wrong to accept medical help in most situations. If they are wrong (as I think they are) they and their children may suffer and die unnecessarily. People have gone to the Philippines believing that "psychic surgeons" there could cure them of serious diseases. As a result they may have neglected medical treatments which could have saved their lives.
Often it is only our money that we lose, but if you are like me, you'd like to avoid that as well. Politicians may mislead us into supporting programs that waste our money, advertisers may convince us to pay more for a product that is not better, cult leaders may get us to contribute money to false causes, and sincere sounding people (some honest, some con artists) may talk us into investing in losing propositions. We all know people that have made mistakes like these.
False beliefs about science can delay finding the cures for diseases or new sources of energy or lead to bad educational or social policies. Because of this, scientists usually take many precautions to avoid accepting findings that are not sufficiently confirmed. Even so, science has sometimes gone off on wrong paths.
It can be very difficult to know if a particular thing is true, such as whether a particular religion is true or whether a medicine is effective or whether a politician will do a good job. But it should not be that difficult to recognize when we do not know something. I believe that people can learn that ability, but unfortunately our schools and our culture as a whole do little to help people develop it. If we care about the welfare of humanity, this is something that must change.