“Laws of the Knowable,” the second of the two first principles underlying his system, Spencer referred to as:

A statement of the ultimate principles discernible throughout all manifestations of the Absolute – those highest generalizations now being disclosed by Science which are severally true not of one class of phenomena but of all classes of phenomena; and which are thus the keys to all classes of phenomena

In other words, from his study of science he had arrived at generalizations capable of accounting for everything, even the Absolute. All the rest about Spencer is footnotes.

In his day Spencer was popularly acclaimed as the greatest living philosopher. He rose from humble origins to become England’s foremost explainer of the meaning of evolution. He was elected a corresponding member of the philosophical section of the French academy of moral and political sciences. New York’s business elite lionized him for identifying them as truly the masters of the universe, for their unwavering application in their business practices of the underlying laws of nature. His acclaim of evolution as one of those laws came a fews years prior to Darwin publishing his “Origins…” and under the catchphrase “survival of the fittest” Spencer went on to become evolution’s primary spokesperson.

Then, suddenly, around the turn from the 19th to the 20th centuries, his light went out and he remains today largely unknown. His System, that he published and updated from 1862-93, is entirely unreadable. I’ve borrowed most of this review from the Internet Encyclopedia of Philosophy (IEP):

Spencer argued that all phenomena could be explained in terms of a lengthy process of evolution in things. This 'principle of continuity' was that homogeneous organisms are unstable, that organisms develop from simple to more complex and heterogeneous forms, and that such evolution constituted a norm of progress. This account of evolution provided a complete and 'predetermined' structure for the kind of variation noted by Darwin…

Again from the IEP:

The purpose of each science or field of investigation was to accumulate data and to derive from these phenomena the basic principles or laws or 'forces' which gave rise to them. To the extent that such principles conformed to the results of inquiries or experiments in the other sciences, one could have explanations that were of a high degree of certainty. Thus, Spencer was at pains to show how the evidence and conclusions of each of the sciences is relevant to, and materially affected by, the conclusions of the others.

Note that August Comte’s ordering of the sciences into a reductionist system, “higher” sciences being interpreted in terms of those more fundamental, had already been grinding on for a couple of decades before Spencer began publishing. Notably, though, Comte’s work does not appear to have been commented on in English until John Stewart Mill summarized it in the 1860s, and it’s still not available in English translation. Charles Darwin was dazzled by a review in English of an early volume of Comte’s work in the 1830s as he was embarking on his search for a mechanism for evolution, and Comte may have inspired him to come up with his reductionist account of the origins of species.

The second half of the 19th century saw the growth of two great contradictory principles. One was Spencer’s idea of Progress--it was according to laws basic to the universe that the simple would progress to the complex. The evolution of living creatures was merely one instance of this fundamental law. The other great principle lay in thermodynamics, which said exactly the opposite—the fundamental direction of things was to go from the more highly ordered to the less highly ordered. Spencer’s fall may be seen as simply the weight of scientific opinion tending towards that second principle.

My final judgment on Spencer: Comte and thermodynamics were the beginning and the end of him.