The thesis of this work is that the modern scientific world view, emerging in the 20th Century, makes a new metaphysics in System Science. That metaphysics illuminates the most difficult subjects in Psychology, like mind, consciousness and the self. It proposes that the phenomena of life and mind-and even of "soul" and "spirit"-are the natural emergents of the basic forms of Nature itself, and that these forms have been discovered and described in the science of the last hundred years. The main resources for this conclusion are in those developments, as amplified in the work of Alfred North Whitehead and Gregory Bateson. The book is intended as a reading in History and Systems of Psychology and for an interested public. The title of the book means "messenger of mind." It is a tribute to Galileo Galilei, whose book, Siderius Nuncius ("messenger of the stars") appeared in 1610 and founded both telescope Astronomy and the modern scientific era. Cheaper b&w and e-book editions are available.