Herbert Spencer (1820-1903) was an English philosopher, best known for his scientific writings. Together with Charles Darwin and Thomas Huxley he was responsible for the acceptance of the theory of evolution. His well-known essay on Education: Intellectual, Moral and Physical was considered one of the most useful and profound books written on education. He projected a vast 10-volume work, Synthetic Philosophy, in which all phenomena are interpreted according to the principle of evolutionary progress. Together with Charles Darwin and Thomas Huxley he was responsible for the acceptance of the theory of evolution. In First Principles (originally published in 1862), the first of the projected volumes, Spencer distinguished phenomena from what he called the unknowable - an incomprehensible power or force from which everything derives. He limited knowledge to phenomena, i.e., the manifestations of the unknowable. He maintained that these manifestations proceed from their source according to a process of evolution.