Hermann von Helmholtz (1821-1894) was a polymath of dazzling intellectual range and energy. Renowned for his co-discovery of the second law of thermodynamics and his invention of the ophthalmoscope, Helmholtz also made many other contributions to physiology, physical theory, philosophy of science and mathematics, and aesthetic thought. During the late nineteenth century, Helmholtz was revered as a scientist-sage - much like Albert Einstein in this century. David Cahan has assembled an outstanding group of European and North American historians of science and philosophy for this intellectual biography of Helmholtz, the first ever to critically assess both his published and unpublished writings. It represents a significant contribution not only to Helmholtz scholarship but also to the history of nineteenth-century science and philosophy in general.