When I was young, I had faith. I believed in certain things, not because of some direct evidence, but because believing was the right thing to do. When I encountered someone who disagreed with me, it was my duty to convince them of the truth, as I was taught it.
As I grew, I embraced skepticism. I reserved belief for things with a rigorous logical basis or peer-reviewed, double-blind, scientific studies. My approach to others was disbelief by default, generally assuming that my skeptical outlook gave me superior access to truth.
However, the more time I've spent the more I've realized: real truth is inaccessible.
Reality is complex. Quantum mechanics implies that, at the lowest levels, reality is either probabilistic or presents an information horizon that we can only model probabilistically. Of course, as humans, we can't perceive that scale anyway. We see Newtonian mechanics and thermodynamics and other emergent phenomena.
For example, it's not precisely true that the coffee I'm drinking has cooled to room temperature. If you had perfect knowledge of the position and momentum of all the molecules in my coffee, you would find none of them have a temperature at all. The atoms are banging into one another, slowly transferring the momentum from some of the fast atoms to some of the slow atoms until, on average, no more momentum is going from coffee to air than from air to coffee.
Even that description is untrue, somewhat. The molecules don't really "bang into one another." In fact, the atom in each molecule perturbs the electron field allowing only certain configurations of electrons. The electrons, when they get close to one another, interact according to Feynman's path integral formulation. They probabilistically exchange energy, jumping from one stable configuration to another, typically absorbing or emitting a photon on the process.
Those are each useful descriptions in our human minds, but they omit a massive amount of detail, and only roughly approximate what's actually happening.
Your mind reduces the impossible complexity of reality into a simple model that enables you to reason about what's happened or will happen.
So you see, everything you believe is wrong.
Where does that leave us? Believe nothing and die!? Perhaps not.
Beliefs have utility. By understanding the relative temperatures of my coffee and the room, plus the second law of thermodynamics, I was able to predict that my coffee would get cold. I even put it in a special cup (a mug) that increases the time-to-equilibrium in the coffee-room system.
It's true that my coffee will get cold, but it's only insofar as "coldness" is a useful way of thinking. One could imagine a more true way of describing the same thing, either by providing more detail, or by replacing temperature with a different abstraction that makes more accurate predictions.
Thus we should judge potential beliefs by their predictive power, rather than as absolutely true or false. Predictive power, interestingly, is also the ruler for truth in the scientific method.
If we measure truth by utility, it becomes a gradient rather than a boolean. My current beliefs can almost certainly become more true than they are now. The disagreeable beliefs of others perhaps contain some truth that can be found.
So first, know that everything you believe is wrong.