Our mind is under ceaseless attack. Constantly, our senses assault it with a gazillion bits of information about the outside world, and the mind itself produces its own fair share of deductions, inferences, and projections on top of that–things we commonly refer to as thoughts. There’s just no way any system anywhere could process all that data in real time, so over eons of evolution, human minds have developed rigorous methods to separate what is considered useful from all the other crap.