When I was a young child, I naïvely believed anything I experienced or that anyone told me as true. As I started adolescence, I started to question that, and realised that people who tell me stuff might be mistaken, or intentionally lying to me. I became very interested in optical illusions, and realised my senses could be fooled too. I had to rely on measurable, repeatable truth that scientific experts had written in pop science books.
Then I thought about simulations, being in a story (like in Sophie's World), gods, and every other possibility that the entire world I experience is not real and is created to test me, to observe me, indifferent to me and I'm there by accident - whichever it was, I couldn't believe for sure that anyone besides me really existed, or anything I knew through my senses. Only my logical reasoning could be trusted. I am doubting therefore I exist, but I couldn't know anything else for sure.
Until recently, I realised when I was ruminating one time, and thinking about which is better: truth or happiness. Most of the times I'd ruminated, I knew I'd come to the conclusion that I'd rather be right than happy. I had logic to back this up, it's more important to know the truth because then I'm happy about being right. But when I'd been happier, I thought being happy was more important than being right - after all, what's the point of being right if it doesn't bring you pleasure, seeking pleasure and avoiding suffering being the whole goal of life?
I realised that what I thought was logical reasoning to support my conclusion wasn't logical at all. It was a rationalisation to support whichever conclusion made me happier at the time. When, for chemical reasons in my brain, I was happy, I wanted to remain happy. So I'd subconsciously convinced myself that I had logic to convince myself that happiness is preferable. When my hormone levels were low so I was feeling down, telling myself that at least I feel better because I know the truth is a way of coping.
And I realised that when my 'logical' reasoning is just a rationalisation for an emotional state caused by brain chemicals and my body, I can't trust any 'logical' argument my brain thinks of. I don't exist because I'm thinking, I exist because I have an innate sense of existing. So therefore, I can't trust anything I think is logical. But wait, that there is a logical statement! So I can't trust it either! And so on... aaaAAARGH!
The more I try to find truth, the less I find I know. I somehow get even more agnostic than I thought it was possible to be, I at least thought, 'Alright, I have no idea what the universe is, but as an external observer I know that I exist.'
I am no longer an external observer! My observations about how my hormones and body affects my emotions, which in turn affect how infuriated I am at the fact that I don't know stuff, that I don't have free will - not the other way around - means I can't even think anymore, as my brain is part of the compromised system. I am compromised.
The more I learn, the less I know.
(I will ignore the context for the sake of argument)
The measurement itself can't be trusted. If you have a 1m stick and you measure it, its still is 1m stick. But if it's moving really fast (close to the speed of light) the stick is somehow shorter. We have made models to predict that but still we can't measure the true length of the stick. We can't know the true length of the stick. Just the approximation within quality of our measurement device.
There are actually two truths: subjective truth and objective truth. We can't really know objective truth but we can get really close to it with our subjective truth.
Subjective truth is really powerful concept as it can be modified by ourselfs. If your subjectivity taints your thinking then change it so it reflects your values.
The philosophy is a study of "How to think" even in the face of hormones and emotions. Thinking logically can be trained and it can be learned. It can also help state right questions and not all questions are logical.
Also not all thoughts come from logic. Some come from emotions or some other place some people call soul. The question "Do I prefer being right or being happy" is not a logical question. It can't be answered logically. The question should be: "Should humans strife for being happy or being right?" Is closer as it can be argued for one side or the other but emotions don't play a huge role in swaying the consensus of arguments.