The composition of our brain of a human part of consciousness and analytical thinking, and a primal brain where the primary life functions are regulated, is the cause of many misunderstandings in our thinking and opinions over all kinds of things.For example, we consider cultural matters like painting and music to be high-quality activities, whereas they are based only on physical-chemical processes that are not properly understood by the human part of our brain and therefore highly assessed. Similarly, our thinking about the meaning of life, joy of life, murder, intelligence, spirituality, religion, love and many other matters is based on believing and not on knowing for sure, as the title of this booklet states.Ninety years of life experience has helped the auteur of this booklet to come to this realization.