Mill was emphatic about the ‘Law of Causation’: it expresses ‘the familiar truth that invariability of succession is found by observation to obtain between every fact in nature and some other fact which has preceded it’. As men of science reiterated, knowledge of constancy of effects is the empirical foundation for scientific progress, and this is independent ‘of all considerations respecting the ultimate mode of production of phenomena, and of every other question regarding the nature of “Things in themselves.”’ In Anglo-American scientific culture, this view of knowledge and scepticism about ‘ultimate’ things came to appear almost self-evidently rational.

Mill, in his dry way, was stating a great deal. When men of science explain something by specifying its causes, he argued, we must be clear that they do not make a claim about a real active agent (or ‘the ultimate mode of production’). Such claims are no business of science; they belong with metaphysics, or perhaps theology, and these areas of thought Mill put to one side.