6 ideas
Spencer argues that both science and religion, pushed to their limits, end in ideas that cannot be conceived: self-existence, infinite space, the ultimate nature of force. He treats this shared inscrutability, the Unknowable, as the ground on which they can be reconciled. Religion keeps the sense of an absolute mystery, and science claims everything that appears.
To think anything is to relate, distinguish, and classify it against something else. An absolute, which by definition has no relations, therefore cannot be known. Yet the very idea of the relative implies a non-relative, so we keep an indefinite consciousness of something beyond appearances without ever grasping what it is.
Spencer defines evolution as matter becoming more integrated while motion dissipates. Along the way it passes from an indefinite, incoherent homogeneity to a definite, coherent heterogeneity. He applies this one formula to nebulae condensing into stars, embryos developing organs, languages multiplying, and societies dividing labor.
Spencer holds that the persistence of force, which we would now call conservation of energy, is the one truth beyond proof, since every proof already assumes it. He then tries to deduce his other laws from it: the indestructibility of matter, the continuity of motion, and the equivalence of transformed forces.
Spencer argues that a uniform mass cannot stay uniform, because its parts are exposed to incident forces in different ways and so change in different ways. Each cause also produces more than one effect, which multiplies differences over time.
In Spencer's view, every evolving system moves toward equilibrium, a balance of forces that ends further change. Dissolution then follows, as integrated matter absorbs motion and breaks apart.