All of our knowledge about everything is incomplete. Every being only ever sees a tiny fraction of reality, and all our logic, emotions and heuristics are instinctive best-guesses. Any theory about reality, from routine physics theories to outlandish religions, rely on the shared experiences of mankind. Yet even if you combine the efforts of millions of people you do not guarantee that in consensus, you find actual truth. The entire Human species has been wrong on major aspects of reality - the Earth is not flat, for example. If mass belief isn't a guide to what is true, then, how can any individual ever ascertain truth, amidst his own personal limits of knowledge?