All I ever know about the world is what I experience. An apple is sweet and crisp, but what is that? Don't we now know that the apple is nothing more than a collection of atoms? How are those atoms sweet and crisp while other atoms are not?
We can even set aside philosophical ideas and appeal to science. We know about quantum mechanics and the mind. We are disconnected from the world by no less than four steps. We don't know what an object is like. We see the light bouncing off of it. We don't know what light is actually like, we only know what our eyes tell us about it. We don't know what our eyes are telling us, we only know how our minds interpret that information. We don't even know what our minds are telling us unless we attend to that information. There is a famous experiment where a group of people are told to count how many times two people bounce a ball back and forth. They focus so completely on this, that almost everyone in the group misses the gorilla that walks by in the background. The gorilla was there, the light bounced off of it, the eyes detected the light, and the mind registered the information, but they didn't “see” it.
This is the very reason why magic works. Our perception can be easily fooled. Perhaps fooled is too negative a term. When we “see” or more accurately attend to information, it's actually rather difficult to fool a human. That's the reason why magicians are so skilled, they need to be. Our brains are very good at understanding the world with the limited information it has.
We say we “know” many things. When I use the word, I will use it in the popular sense and explicitly state when I'm using the word in other ways. For example, we know something if we read it in a respected magazine or journal, or if we see it with our own eyes.
I don't necessarily consider belief as an idea without reason, such as a belief in God. I believe my friend is at work right now. I have very little reason to think this, but he's usually been there at this time in the past, but it's well within reason to think he called out and is instead in my kitchen eating all my food.
Faith could be considered an extreme degree of belief, where there is basically no reason for believing. But even that is open to interpretation. I have faith that if I was trapped in Bolivia with no money, my friend would send me money for a ticket. That situation has never happened, and he's never needed to do something like that for someone else, but his character seems to indicate that he would do it.
I actually count belief in God as the only true baseless belief.