Herbert Spencer intended First Principles to be an introduction to his comprehensive study of the world, entitled The Synthetic Philosophy (1862-1896). However, he made it an independent work, complete in itself, that not merely announced the principles of evolutionary naturalism but illustrated them amply with examples from all fields of knowledge. For good measure, he also raised the issue of science and religion and proposed an amicable solution.
Spencer shared the classical positivist conviction that knowledge consists solely of empirical generalizations or laws. Particular sciences, he held, have the task of formulating the laws that govern special classes of data; however, inasmuch as there are phenomena common to all branches of knowledge, a special science is needed to gather them up into laws. This, he claimed, was the business of philosophy. In his view, that business was now completed. The synthetic philosophy included not only general laws but also one law from which all other laws, both general and specific, could be deduced a priori. He therefore offered a new definition of philosophy: It is “completely unified knowledge.”