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.

Uniformitarianism and the History of Geology

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Uniformitarianism is one of the most basic principles of modern geology, the observation that fundamentally the same geological processes that operate today also operated in the distant past. It exists in contrast with catastrophism, which states that Earth surface features originated suddenly in the past, by radically different geological processes than those currently occurring. Note, however, that many "catastrophic" events are perfectly compatible with uniformitarianism. For example, Charles Lyell thought that ordinary geological processes would cause Niagara Falls to move upstream to Lake Erie within 10000 years, leading to catastrophic flooding of a large part of North America.

Uniformitarianism is a generalisation of the principle of actualism, which states that present day-processes (astronomical, geological, paleontological,...) can be used to interpret past patterns. It is also known as "the present is the key to the past". The principle of actualism is the cornerstone of paleoecology.

The geologist James Hutton was a pioneer of the principle, which was later popularised by Charles Lyell and influenced Charles Darwin. In the 18th and 19th centuries, the debate between the two theories was intense, since uniformitarianism seemed hard to reconcile with the prevailing religious beliefs of the time. Today, however, most if not all mainstream scientists support uniformitarianism as do most mainstream religious denominations.

The understanding of slow geological processes have changed in several ways. Before continental drift (see plate tectonics) was recognized in the 20th century, the surface of Earth was believed to have remained generally unchanged since creation. Cooling from a molten state was believed to have caused shrinkage, which caused mountains and folding of the surface. Currently it is accepted that much of the mantle is plastic and fluid, and the crust is slowly moving over it. It is this relative motion that produces folding, compression, rises, depressions, etc.

In recent decades, the theory of uniformitarianism has been modified to reflect the discovery that catastrophic events occur today and have occurred in the Earth's past. The present is still the key to the past, meteorite impacts, giant earthquakes, tsunamis, and supervolcanic eruptions occur today as they have in the past and these events provide punctuations in an over-all gradual process.

Geology | Philosophy of science | Evolution

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