When you stop believing in authority how do you decide what to believe and what not to believe? The method has been emerging slowly since the twelfth century. The only valid way of demonstrating truth in an explanation is by making predictions from it. If these are repeatedly true and others can verify them then your original prediction must contain a level of truth. This book traces the development of the equipment which allows us to make these predictions and outlines some serious traps we have fallen into in so doing. When we downgrade thought systems which fall short of this method we meet some surprising consequences.