Our ability to make rational decisions is limited by how much information we can gather and process within the amount of time we have available. We can only process so much in our minds at one time. So, we estimate; we guess; we assume; we do the best we can. Few decisions we make can be considered purely rational.
To the degree we guess, we are exercising faith. We are putting our trust in something we are not sure about. Even major scientific advances are built on strong elements of faith. If we ask "Why?" enough times, eventually even scientists will say, "Well we don't know for sure, but this is what we assume..."