Just how deep do you believe?
Will you bite the hand that feeds?
Will you chew until it bleeds?
Can you get up off your knees?
Are you brave enough to see?
Do you want to change it?
--Nine Inch Nails
What is the purpose of the human brain? What function does it serve? Be careful; this is a trick question!
If you say "The brain is an organ of thought" or "The brain is an instrument of knowledge" or "The brain is the way we understand the world," that's the wrong answer. The correct answer is that the brain is an organ of survival. We have these big brains because they enabled our ancestors to survive; in that sense, they are no different from claws or fur or fangs.
And like all organs of survival, the brain was shaped by natural selection, sculpted by evolutionary pressures that favored the traits that helped our ancestors survive. The big brains we have now were molded and shaped to one purpose: to help small bands of hunter-gatherers survive.
Back in the day, when we rarely lived longer than 20 or 25 years and starvation battled with predation by large predators for the top spot spot in the list of "things that killed human beings," our brains gave us a competitive advantage. They did this in part by acting as engines of belief, allowing us to form models of the world and create beliefs about the world that gave us an edge.
For example, an early human who observed that if he was upwind of his prey, the prey got away, but if he was downwind of his prey, he could more easily kill it formed a belief: "Staying downwind from the prey makes it more likely that the prey will not escape."
Of course, other animals know these things instinctively. But the advantage of our big monkey brains is that we do not have to rely on instinct; we can form beliefs on the fly, as we go along, which means we can function in environments our instincts are not prepared to deal with. The brain as an organ of survival allows us to make observations and draw beliefs from these observations, and these beliefs give us a competitive advantage.
These beliefs can be immediate and concrete, such as "If I stick my hand in the fire, it will hurt." They can make predictions about the future, such as "The sun will rise tomorrow" or "If the days grow longer and the weather grows colder, then winter is coming, and food is about to become less plentiful." A belief can be negative, such as "If I leap from the top of this tree, I will not be able to fly."
Having a brain optimized for forming beliefs is important if forming beliefs is your survival strategy. If you think of the brain as a belief engine, which can either believe something or disbelieve it, and if you think of a particular belief as being true or false, it is easy to construct a game theory matrix describing all the possibilities, with two success modes and two failure modes:
Ideally, our brains lead us to believe things that are true, such as "A large leopard is a dangerous adversary," and to disbelieve things that are not true, such as "I can eat rocks." But there are two failure conditions as well: rejecting beliefs that are true, and accepting beliefs that are not.
The failure conditions have survival implications. Believing untrue things and not believing true things can both lead to disaster.