Growing up made easier
Truth is what is. In other words, a statement is true to the degree to which it corresponds to reality1.
Things that exist are true, because they are what is. They exist in reality. Statements about things are true if they describe the things as they are in reality. This seems painfully obvious, but here's where things get tricky: there are two kinds of truths: subjective and objective.
Subjective truth is that which is true for a certain person (the subject). It's their personal feelings, thoughts, opinions, and preferences. In this blog, we will avoid subjective truth. Why? Because there's no reason to debate subjective truth; differing opinions are seldom resolved by debate. For instance, you can't just change someone's favorite color just by arguing with them. (What makes red better? It just is, someone might say).
Objective truth, on the other hand, is that which is true independent of a person. It describes external realities (about objects) which are real regardless of what someone thinks or feels about them. For instance, there are 100 centimeters in a meter, whether someone likes it or not. This is important: even if someone feels like there aren't 100 centimeters in a meter, or even if someone doesn't know that there are 100 centimeters in a meter, or even if someone disagrees that there are 100 centimeters in a meter, there are still 100 centimeters in a meter. Period. That is an objective truth for everyone (note: independent of a person).
Why is it important to distinguish between these two? Because there are some people who ignore or disagree with objective reality. There are people who believe (incorrectly) that the earth is flat (it's not). Does that mean that the earth is flat? No, it just means that those people are wrong. It is not possible for two people to disagree about objective truth and both be right.
So perhaps the best way to know objective truth is to consider science? Certainly science is one way to know truth: we propose a hypothesis, we test that hypothesis by collecting data or conducting an experiment, and we analyze that data to see whether to accept or reject our hypothesis. But science can't be the only way to know truth. Most of math, for instance, isn't science; we don't conduct experiments or collect data to see if two plus two is four2. Or consider historians, whose job it is to study the truth of what historical events. Likewise, historians are not scientists, and do not perform experiments to determine what events transpired. Clearly science cannot be the only way to know truth. Indeed, the statement "truth only comes from science" is self-contradictory: what experiment can you run to prove that truth only comes from science3?
Perhaps instead truth comes from the Bible? Certainly some truth comes from the Bible (more on this later), but it is equally contradictory to say that all truth comes from the Bible. If it were true that all truth comes from the Bible, then the Bible would have to say that all truth comes from the Bible (which it doesn't)4.
Or perhaps instead there is no objective (universal) truth after all? This seems to be a popular way to act nowadays (usually unknowingly, we should add): "what's true for you is true for you, and what's true for me is true for me", or "don't impose your truth on me". But this is yet another third contradiction: is it true for everyone that there's no truth for everyone?
Or perhaps what's true is what the majority of people believe? Clearly this also isn't the case. Even if everyone on earth suddenly believed that the earth was flat, the earth would not suddenly become flat.
So then how do we know truth? As Catholics, we believe that faith, science, and reason all contribute to knowledge of truth (but more on that later!).