In any given situation we assume that people, including ourselves, will act sensibly. But that is not an assumption on which we can rely. We live our lives and make our decisions on much shakier foundations than reason and good sense. It is important to understand that people seldom think logically and therefore very often do not act rationally.
The long testimony of history, and all the researches of behavioural scientists, clearly indicate that the cool exercise of logic is only one influence in how people think and therefore act. There are other influences at work which are all too often more powerful.