The study of social hygiene means the study of those things which concern the welfare of human beings living in societies. There can, therefore, be no study more widely important or more generally interesting. I fear, however, that by many persons social hygiene is vaguely regarded either as a mere extension of sanitary science, or else as an effort to set up an intolerable bureaucracy to oversee every action of our lives, and perhaps even to breed us as cattle are bred. That is certainly not the point of view from which this book has been written. Plato and Rabelais, Campanella and More, have been among those who announced the principles of social hygiene here set forth. There must be a social order, all these great pioneers recognized, but the health of society, like the health of the body, is marked by expansion as much as by restriction, and, the striving for order is only justified because without order there can be no freedom. If it were not the mission of social hygiene to bring a new joy and a new freedom into life I should not have concerned myself with the writing of this book.