Suppose your are literate in a precise subject like the mathematical or natural sciences. Suppose you know how the communicative property works, how to factor polynomials, or the formulation of the quadratic equation. Suppose you know that atomic, relativity, or evolutionary theory are true beyond any reasonable doubt, that plate tectonics occupies a fundamental place in modern geology, or that the most recent report of the IPCC (the definitive international body on climate science made of thousands of climate scientists) says that the probability that humans are the main cause of global warming since the mid 20th century is between 95% and 100%.

Now suppose you encounter skeptics who doubt these scientific ideas. You are clearly right and they are scientifically illiterate about the scientific theory in question. (With the caveat that no knowledge is absolutely certain.) But what difference does it make? In a sense it doesn’t seem to matter. They may get along better with their false beliefs than you do with your true ones. Whether they are flat earthists or climate change deniers they may be happy in their beliefs, and changing their mind may cause them cognitive dissonance.

But in another sense the truth does matter. If we want to build a bridge we will need mathematical principles; if we want to understand flu viruses we need evolutionary theory; if we want to find oil we need geology; if we want to make chemicals we’ll need to understand the periodic table; and if we want to understand climate change we need to know basic physics. It may not matter if people privately believe they can find oil by using tarot cards, build highway bridges out of duck tape, or cure disease with incantation; but if you really want to find oil, build secure bridges or fight disease you’ll need geology and engineering and modern medicine.

Of course this may all seem obvious because the mathematical and natural sciences are so precise. But what of less precise sciences? If we turn to social sciences like economics, psychology, history, or political science the situation is a bit different. In these fields even the experts sometimes disagree. I can say with certainty that there was a Roman Empire or a Holocaust in 20th century Europe if I’m a legitimate historian, but exactly why they happened is open to debate. Still a lot hinges on these disciplines—many lives are affected by the—so it important to find out what’s really true regarding their subject matter, not just what we want to be true. We must proportion our assent to the evidence, view the matter impartially—very hard to do given human psychology—and then act the best we can.

If we get to subjects like the humanities and aesthetics we are in the realm of relative, or nearly relative, truth. The truths about philosophy and religion, even if they are objective, are so difficult to discern one often must accept disagreement. And when we get to what is a good movie, book, poem, or piece of art, the truth does seem subjective and relative. There just isn’t much point in fighting about whether broccoli really tastes good.

Other than where it seems there is no truth—the broccoli case—the truth certainly matters. And not just for public policy. If individuals believe falsehoods it may cost them money. If they think they can beat the odds in Las Vegas or that clairvoyants or palm readers can predict the future, they will pay for these false beliefs. False beliefs might even cost you your life. You may die in unjust wars because you believe the lies of politicians, or you may fail to wear a seat belt because they would rather be “thrown clear in an accident.” (This was actually a widely held belief in the early days of seat belts in cars. I am not kidding!)

Thus we must distinguish between knowing what’s true and convincing others that something is true. Both are difficult. The first results from using the scientific method, from a careful and conscientious examination of the world. Scientists toil for years in their laboratories teasing a bit of truth out of reality and adjusting their beliefs on the basis of the evidence. (For more see Charles Sanders Pierce’s classic: “The Fixation of Belief.”) Convincing others is much more difficult, especially since many people cling to comfortable beliefs and intuitions, avoid cognitive dissonance, or are simply enjoy being contrary, argumentative, and disagreeable. Add selection bias and the various reasoning errors that human are so prone too, and it easy to see why it is difficult to change a mind.

In the end we should continually reexamine our own beliefs—to rid ourselves of false ones–and state the case for those things about which we have great certainty—well-tested scientific theories for example. After that there isn’t much we can do except hope that truth will win out in the end. This doesn’t mean I’m optimistic about this happening. I just believe that if the truth doesn’t matter, then not much else does as well.