Definition of Uniformitarianism is the principle, articulated by nineteenth century British geologist Sir Charles Lyell, that geological history can be explained by reference to presently observable processes. In his now-classic work, Principles of Geology (1830), Lyell argued that past geological processes were the same in rate, intensity, and kind as those currently in operation, and earth history was not progressive (substantive uniformitarianism). Many geologists were unconvinced by Lyell's arguments for constant rates and intensity, but enthusiastically adopted uniformitariansm as a methodological program of interpreting the geological record by analogy with modern environments and events (methodological uniformitarianism). In the twentieth century, unofrmitarianism was often said to be the fundamental guiding principle of modern geology.