Uniformitarianism, in the philosophy of science, is the assumption that the natural processes operating in the past are the same as those that can be observed operating in the present. Its methodological significance is frequently summarized in the statement: "The present is the key to the past."
Uniformitarianism is most closely associated with geology, but it is also used in astronomy, paleontology, and other sciences whose objects of study are in the past and (as a result) beyond the reach of direct observation. Uniformitarianism has its philosophical roots in antiquity, but it was refined and popularized by British scientists of the late 18th and early 19th centuries: notably James Hutton, John Playfair, Charles Lyell, and William Whewell (who coined the term).
Uniformitarianism, though often treated as a single idea, is in fact a family of four related (but not identical) propositions.
Paleontologist and evolutionary theorist Stephen Jay Gould usefully characterized them, in a 1962 paper, as: 1) Uniformity of law; 2) Uniformity of kind; 3) Uniformity of degree; 4) Uniformity of result. The first sense of uniformitarianism was almost universally accepted, and the fourth almost universally rejected, by Western scientists from the mid-19th century onward. The second and particularly the third senses remained controversial and (though more increasingly accepted in the 20th century) have been occasionally challenged by scientists who believe the presumption of uniformity (in the second and third senses) is unwarranted.