Can we ever really know anything?

How many times throughout your day do you say that you know? I know. I know that you are standing in front of me right now. I know what I see. I know even what I dont see. These are all things that I say and that I have heard from other people. But, how do really really know? If our minds are our own how do we know what we think we know is true? Our lives are shaped by what we experience externally, and how we react to them internally. We all live in our own worlds that are unique and all very very different from the next. I know that this is air I am breathing and this is a computer I am typing on, but sometimes when I dream I think that I am doing something that I not really doing. Is it up to a matter of faith? Is it a matter of fact? Are the two ideas combined to create what we know to be true?