Charles Lyell’s philosophy of science required uniformity of law, kind and degree as methodological (a priori) assumptions. Many of his contemporaries argued that the latter two, especially uniformity of degree, were substantive claims to be determined a posteriori. Some historians of science continue to reprove Lyell for confounding methodological assumptions and substantive claims in his “principles of reasoning” in geology. Lyell adhered to a philosophy of science most authoritatively articulated in his time by the astronomer John F. W. Herschel (1792-1871). His strict interpretation of Herschel’s version of the verae causae doctrine necessitated uniformity of kind and uniformity of degree. These methodological assumptions placed severe constraints on Lyell that he loosened by using what William Whewell (1794-1866) termed the law of continuity and the method of gradation to extend “now” into the remote geologic past. He clearly recognized the concept of recurrence interval and allowed for catastrophes so great as a “tremendous deluge” resulting from the sudden draining of Lake Erie, the creation in a few months or at most a few years of an inland sea covering more than 150,000 square miles in Central Asia, and near instantaneous movement of more than 3,000 feet on a fault—all in the regular course of nature!

Lyell believed that known processes operating at present intensities could effect enormous changes either when summed over long periods of time or suddenly when acting in unique situations. For him, failure to arrive at satisfactory explanations for “former changes of the earth’s surface by reference to causes now in operation” was more likely to indicate a lack of knowledge than a failure of method. Lyell allowed the intensity of “causes now in operation” to vary to almost any degree so long as the variation was cyclic, not directional. Herschel cited Lyell’s theory of cyclic changes in climate as a vera causa and dismissed directionalist explanations as “not real causes in the sense here intended.” Lyell may have been wrong, but he was not confused. His philosophy of science required uniformity of degree as an a priori assumption.