In this book, Alfred Sauvy studies the frightening gap between the mentality of the man of action and the man of science. He shows that, in this century which inherits of two centuries of hard sciences, the "man of action" only acts in an empirical or passionate way. People have no objective information about facts. Economic and social myths still dominate their will. Time and again the notion of what is right prevails over the notion of what is possible, and desires prevail over realities.