Seeing as pursuing truth is our objective, a natural question is how do we come to know truth? Perhaps even more generally, what does it mean to know? We have an intuition of what it should mean to know, namely that a given idea in our head, which we believe corresponds to some actual reality, truly does correspond to it. If we look before us and see a chair, and then consider it, we can say that we know the chair, or at least, that the chair exists. Should we desire to know more about the chair, what approach should we use? We could look to those around us and inquire about the chair, receiving the information that they give us as genuine knowledge about it. Alternatively, we could investigate the chair quite directly. Do we desire to know its mass? Perhaps lift it and feel its resistance to gravity. Maybe instead you wish to know the material? Knocking on it with your fist should make a distinct sound for each kind of material that it could have been made from. Having an idea of its material and its mass, you might predict whether, if you sat in the chair, whether it would collapse under your weight, or if it would hold you stably. We can see from these that there are two channels by which we may receive information and discern knowledge from. The first is the top-down approach, wherein one who has knowledge or authority on a matter passes down information to us, and we accept it as though they were telling the truth. The second is the bottom-up approach, where we don't put our faith in authority to tell us what is true, but we instead observe the world directly, make inferences from it, and eventually build up our entire ideology from that one blank slate.

Getting back to the chair and whether it would support your weight, even if you have an idea of whether it would support you, how can you be certain? Surely you would just have to sit in it and find out. But can you know in advance without putting yourself at risk? After all, if you're wrong, and it doesn't support you, you would be in for quite a nasty fall. But suppose you sit and it does support you. Does this mean it would support you tomorrow or the day after? How can you know for sure? What does it mean to be certain at all? Surely, if a thing were certain, then it could not conceivably be any other way. For instance, if we could prove a thing, if it were logically guaranteed such that the inverse would lead to an absurdity, then that thing should be certain.

Suppose we have as a premise, if it is raining, then it is cloudy. Or in other words, R → C (read R implies C), where R means that it is raining, and C means that it is cloudy. Suppose we also happen to know that there is not a cloud in the sky. If we accept modus tollens into our reasoning process, P → Q, ~Q, ∴ ~P (read P implies Q, not Q, therefore not P), then seeing as it is not cloudy, we know also that it is not raining. After all, as per the initial premise, there have to be clouds in the sky in order for it to rain, right? Or do there? What is to stop a clear day from raining? Doesn't the rain come from the clouds? But how do you know that? Is it because every time that you know of that it has rained, there were clouds in the sky? What is to say that it will be that way tomorrow? Must tomorrow be like today? Even if every day since the beginning of days has been the same, why can't tomorrow be different? Truly, we do not actually know whether tomorrow will be the same as today. We have assumed, given that every day thus far has been consistently the same, that every day from now on shall follow suit. We have assumed that because every rain that we have ever recorded happened when there were clouds, that clouds shall forever be necessary for rain. We have even assumed that the rule of inference, modus tollens, applies to our reality. We have assumed that our reality is consistent and comprehensible to us; and so we assume also that if a thing is absurd, that it must therefore be false. Even the chair that we saw before us, we have assumed that because we saw it, it therefore exists. Do we even exist? How do we know? Because we're thinking? How do we know that we are thinking? Does a thing thinking imply that it exists? We could invent a category of existing to require that if a thing thinks, that it must therefore exist. But does that category correspond to reality, or is it all in our heads? Is all of this in our heads? Is our perception all that is?

To any of us who, so to speak, have our heads screwed on straight, the answers to these seem quite obvious. Of course we exist. Of course the things we observe are real. Of course this world is consistent. Though these are things that we must ultimately assume, these are, as far as we can tell, reasonable assumptions. Despite all the philosophical dialogue, beyond this discourse, we have lives that we live every day. If not for these assumptions that we make, our every day lives would be impossible. Even in our systems of formal reasoning, we must assume certain axioms, and define certain rules of inference, that we assume apply to whatever it is that we are attempting to describe.

But why should we settle for assumptions? Why not insist that we won't believe anything unless it is absolutely proven? But if nothing at all, not even our own existence, can be proven apart from assumptions, why believe in anything at all? One will quickly find in attempting to make no assumptions just how inevitable assumptions are to us. These assumptions seem so obvious to us, as if they were built into our very minds. When you see a chair, did you decide that you would believe in its existence? Or is it rather by instinct that you believe by seeing without thinking about it? Who gave you these instincts? Did you teach yourself how to see? Certainly not, for you are not self-made. You simply woke up one day in a world where you have no idea what's going on, and a whole host of strange instincts that guide your thoughts and actions, despite being unable to account for their existence. Try as the epistemologist may to build up his whole mental world on the uttermost bedrock, not relying on anything taken for granted, that very bedrock is not the work of his hands, but was laid down from above. Even the most staunch bottom-up approaches will eventually collapse into the top-down when all things are tested.

So what does this mean for us, that we mortals are bound to assume the very foundations of our knowledge and take it for granted? First, it shows that all knowledge is faith, and received in faith. Second, seeing as not all assumptions are created equal, it is important to be aware of the assumptions we make, especially if we desire to know the truth. Some of our assumptions are such that they can be tested. These should be tested, both against observed reality and against each other. While we converse with one another, we should expose our assumptions to each other, that a difference in assumptions may not too severely disrupt the discourse. Can two agree unless they assume the same things? Furthermore, this insight compels us to be humble, and not demand absolute certainty as a prerequisite for belief, for nothing is absolutely certain. And this is no fault of the things themselves, or of reality being confusing, but of our minds being capable of doubting even the most obvious propositions. So let us take a balanced approach to things, not being carried away to assume things because we have to, but also maintaining a reasonable skepticism, not demanding a certainty which is impossible for us.