 Spencer’s output was vast, covering several other disciplines besides philosophy and making it difficult to make sense of his philosophizing separate from his non-philosophical writing. And there is so much Spencer to make sense of, namely many thousand printed  pages.[1]  Besides ethics and political philosophy, Spencer wrote at length about psychology, biology and, especially, about sociology. Certain themes, not unexpectedly, run through much of this material. Coming to terms with Spencer and measuring his legacy requires expertise in all of these fields, which no one today has. Notwithstanding this caveat, it seems fair to say that next to ethics and political philosophy, Spencer’s lasting impact has been most pronounced in sociology. In many revealing respects, the latter grounds and orients the former. Hence, it seems best to discuss his sociology first before turning to his moral and political theory. But taking up his sociological theory, in turn, requires addressing, however briefly, the elemental axioms undergirding his entire “Synthetic Philosophy,” which consisted of The Principles of Biology (1864–7), The Principles of Psychology (1855 and 1870–2), The Principles of Sociology (1876–96), and The Principles of Ethics (1879–93).