Everything that we claim to know about reality is built on a stack of presuppositions. As an example, let’s try to prove anything about the physical world. In order to say that we know something about the physical world, we must assume that our mode of experiencing it (through our five senses) is an accurate reflection of it. For example, I believe that I am sitting in a chair because I trust my experience of seeing the chair, and I trust my experience of feeling the chair below me.

We can extend this line of thinking to, well, thinking itself. When we think, we assume that we have the ability to use logic to reason about our experiences to determine true vs false. C.S. Lewis uses this idea to make a case for the existence of a creator:

“Supposing there was no intelligence behind the universe, no creative mind. In that case, nobody designed my brain for the purpose of thinking. It is merely that when the atoms inside my skull happen, for physical or chemical reasons, to arrange themselves in a certain way, this gives me, as a by-product, the sensation I call thought. But, if so, how can I trust my own thinking to be true? It’s like a random arrangement of a printing machine—it can’t be trusted to produce truth.”

The assumption that our brains can be trusted to produce truth is self-evidentially true, since to reject it would be self-defeating. One who does not trust their brain to produce truth cannot even trust the statement “I do not trust my brain to produce truth.” We must assume that our brains can produce truth, otherwise we cannot know anything about the world.

Thus far, we’ve seen that all of our knowledge about the world depends on multiple assumptions regarding the nature of our experiences. It’s entirely possible that our perception of the world is simply a product of deception. However, there is one thing that we can be sure of: the fact that I experience.

Notice that I didn’t say that we can be sure that we experience; the only experience that we can be certain of is our own. Though I can never be certain of the origin of my experiences, I can be certain that I am experiencing. This necessitates one other truth: I exist.

This is exactly what René Descartes’ famous philosophical statement, “I think, therefore I am,” is getting at. It acknowledges that thoughts necessitate existence. I propose that experiences in general necessitate our existence, not just thoughts. In other words, we can be certain of any experience, whether it be pain or emotion, not just thinking. That’s why I’d humbly restate René Descartes’ revelation as “I experience; therefore, I exist.”

We’re not done here. In order to say that anything is either true or false, we must rely on one last thing: logic. Logic, at its core, is the idea that something is either true or false (Law of Excluded Middle), and that true can never equal false (Law of Non-Contradiction). Without logic, there is no such thing as knowledge, since there is nothing to be known. Even the statement, “Logic does not exist,” is inherently a logical statement, since it operates under the assumption that the statement is either true or false. If there is no logic, there is no truth, and without truth, there is no reality. I believe that we can show that the existence of logic is a necessity. How could we prove the existence of logic without using logic? Let me explain.

Because truth can only exist in a logical reality, we can use the existence of truth to prove the existence of logic. If even a single truth exists, logic necessarily exists. Remember that we know at least two truths for certain: “I experience,” and “I exist.” Therefore, since truth exists, logic exists. That’s why I don’t consider the existence of logic an assumption – it necessarily flows from our existence.

So, we’ve finally reached the bedrock of knowledge. There are really only three things that we can know for sure: I experience, I exist, and logic exists. Can we know anything else? We can more, just not with absolute certainty. Instead, we should seek to make only the most justifiable assumptions. In fact, we have very good reasons to make most assumptions.

One of the biggest factors that plays into our beliefs is our past experiences. For example, we are completely justified in believing that the physical world exists since the totality of our experiences has always been consistent with its existence.

Another factor that plays into our beliefs is reason. Reason is the process of interpreting evidence and ensuring our beliefs remain coherent with one another. For example, if I see rain, I conclude that there are clouds. This is reasonable because it fits with my other belief that rain comes from clouds.

There are many factors that shape our beliefs, but one of the most influential is our feelings. Feelings don’t provide truth directly, but they shape how we interpret evidence and experiences. They affect both the strength with which we cling to certain beliefs and how open we are to new beliefs. For example, a fear of heights can make me cling to the belief that “airplanes are dangerous,” and resist the belief that “airplanes are safe.” Even an abundance of evidence can be overridden by feelings, no matter how unreasonable. Our feelings can get in the way of trusting in evidence.

In conclusion, there are certain truths that lie at the foundation for all our beliefs: “I experience,” “I exist,” and “Logic exists.” Each subsequent belief must be evaluated based on reason and past experience. We must be aware of how our feelings affect our held beliefs and strive to hold the most justified beliefs. Our beliefs matter, because everything we do flows from them.