a scientific basis. If those who know why and how neglect to act, those who do not know will act, and the world will continue to flounder. The whole history of mankind and especially the present plight of the world show only too sadly how dangerous and expensive it is to have the world governed by those who do not know. In paying the price of this war, we have been made to realize that even the private individual can not afford to live wrapped up in his own life and not take his part in public affairs. He must acquire the habit of taking his share of public responsibility. This signifies that a very great deal of very simple work, all pointing in the direction of a greater work, must be done in the way of educating, not engineers and scientific men only, but the general public to cooperate in establishing the practice of Human Engineering in all the affairs of human society and life. In writing this book I have had to wrestle with tremendous difficulties in expressing new thoughts and in indicating new methods. The reader who stops to criticize words or expressions because of their more or less happy or unhappy use will miss the whole point of the work. The reading of it should be done with a view to seeing how much can be found in it of what is new and good that may be elaborated further, and put into better form. This new enterprise is too difficult and too vast for the unaided labor of one man—life is too short. The method used in this book in analysing life phenomena is essentially an engineering method, and as physics and mechanics always suggest to mathematicians new fields for analysis, it is not improbable that Human Engineering will give mathematicians new and interesting fields for research. The humblest rôle of mathematicians in Human Engineering may be likened to that of “Public accountants” who put _in order_ the affairs of business.