There are those who think there is one immutable Truth and we believe this Truth because it is true. I believe that this Truth is true because we believe it. My existance is a manifestation of my belief of my existance. I exist. Because of my perception of my existance, I believe that I exist. Because of my belief that I exist, I exist. Because of my existance, I perceive my existance and on and on. It is a self-perpetuating cycle, a feedback loop in the infinite.
