ABSTRACT
Charles Lyell’s philosophy of science required uniformity of law, kind and degree as a priori methodological assumptions. Lyell adhered to a philosophy of science most authoritatively articulated in his time by the astronomer John F. W. Herschel. 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, which he loosened by using what William Whewell termed the method of gradation to extend “now” into the remote geologic past. Lyell believed that known processes operating at present intensities could effect enormous changes either when summed over long periods of time or when acting in unique situations. He clearly recognized the concept of recurrence interval and allowed the intensity of “causes now in operation” to vary to almost any degree so long as the variation was cyclic, not directional. Lyell may have been wrong in assuming uniformity of degree, but he was not confused. His philosophy of science required uniformity of degree as an a priori assumption. 
