There is a flaw at the root of any pursuit of real-world knowledge.
Reason may construct bounded realities, in which all parameters are known with certainty, and within these bounded realities those capable of reason are able to come to conclusions. But the certainty of those conclusions, assuming the chain of logic leading to them is without flaw, are dependent on the certainty of the parameters of this manufactured reason-space.
This is the difference between deductive and inductive reasoning. Deduction is hard, black-or-white logic, and it will find you a certain conclusion, but requires perfect foundations to do so. Induction looks at the information available and suggests the most likely conclusion, but can never be certain.
Thus we are brought to what is known as the ‘brain in a vat’ problem.
How can you know, with that same certainty that you know you exist, anything about the specific characteristics of your existence? Through what means can you conclusively, utterly rule out the possibility that you’re in the Matrix? Or that you are a titular brain in a vat, without even a ‘real body’ outside this simulation, and all your sensory input and motor output just go through a highly effective computer simulation to maintain your perception of a non-simulated reality. How can you rule out the possibility that you don’t have a human brain at all, and are in fact entirely digitally simulated? How can you know whether or not you came into existence about 22 seconds ago, complete with algorithmically populated memories, a program running on some 26th century Clarketech computer written up by that future’s equivalent of a high schooler for an offhand extra-credit project in their programming course? At most extreme—how can you know if you’re the only thing in all of reality, hallucinating a life that reflects no true arrangement of matter?
You can’t rule it out. Which, if you haven’t come to these conclusions before, is existentially disconcerting and do feel free to take a moment to process. Remember that you exist, and as I’ll get into later, you are (presumably; assuming you have good reason to believe so) a person, and that your exact configuration does not impinge on these facts. But I don’t have much more balm for you than that. There’s no easy out. Philosophy has not yet offered a widely accepted or conclusive alternative.
So what we are left with is to either operate purely in the world of inductive reasoning, or to make assumptions for the sake of deduction. I rather prefer the latter, as it posits the possibility of absolute truths beyond just the one. But most anything we try to argue will be rooted in our experiences, and requires assumptions about the world. Even such fundamental ones as “the physical substrate of ‘me’ is as I perceive it” or “there is anything besides my perception.” And most of our conclusions need far, far more than just that.
Deduction then becomes an exercise in contingent knowledge. Given this or that assumption, one can know further things. And my and others’ hope is that, if given enough time, enough interdependence through an enormous arrangement of contingent truths, likely beyond the capacity of the human mind to comprehend at once, we may be able to find more circularities, dependencies, other absolutes. We may narrow down knowledge to a narrower and narrower branching tree, and perhaps one day we will be able to know so, so many more things with true certainty.
Philosophy, being concerned in almost all cases only with contingency, is then pursuit of intellectual consistency. And wisdom, being concerned with consistent correctness, is its attainment.
I aim to be wise.