I have never debated this exact topic on these forums before, but certainly on others. From a purely hard agnostic point of view you are correct, we cannot know anything for sure, just because gravity has kept us from flying off the face of the planet for as long as history has been recorded doesn't necessarily mean it's going to hold true tomorrow. Just because we see our reflection in the mirror, walk around and live our lives, doesn't mean we're not in some celestial dream and that none of this is real. And you are exactly correct, you can't dwell on these things or you'd drive yourself bonkers. So at that most basic of levels, we use faith to believe that those things around us that are demonstratable and proveable and that are confirmed by others are fact. But there is a big difference between 'believing' that 2 + 2 = 4 and that it always has and always will, and believing in something that has no supporting evidence, something that is completely conjecture. Like God for instance. Using a theistic notion of the word, God cannot be touched or seen or registered with any of our physical senses, he was created thousands of years ago as a sort of thesis on how we came to be here and what our purpose is. But there is no supporting evidence for that, you can't prove him with math, there is no 'god variable' required to calculate the effects of the gravity holding us down. There is a lot that we don't understand about science but so far God has not been required for anything to work, and we haven't learned everything yet but there's no foreseable need for god to exist to explain anything. So when I say faith, I am talking about faith in that which is not proveable, not those things which we must accept in order to exist.