“Your attitude determines your reality.” “Fear is the only thing holding you back.” “Gratitude is the key to happiness.” Self-help is full of such pithy statements. I bet you’ve seen hundreds of them. They’re usually presented as great insights, key ideas, or even as fundamental principles of the universe. Almost all of these ideas can be shown to be false with just a little analysis. But before we dismiss them all, we need to delve into an arcane-sounding question: What do we really mean by ‘true’? This question is at the core of epistemology — a long-standing branch of philosophy. And though it may seem incredible, there are at least half…