This book is an elementary exposition. It contains no more technically than seemed readily understandable by the intelligent layman and the medical student desiring a merely general introduction to modern views on the motives of human conduct and the mental processes of which that conduct is the expression. Part I gives some account of processes and motives that are universal and therefore normal. Part II is written from the angle of the physician who sees the results, always common but nowadays more frequently discussed, of the miscarriage of the normal development of human beings as such.