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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."

It was originated by Scottish naturalists in the late 18th century, starting with the work of the geologist James Hutton which was refined by John Playfair and popularised by Charles Lyell's Principles of Geology of 1830. The term uniformitarianism was coined in 1832 by William Whewell, who also coined the term catastrophism for the preceding idea that the Earth had been created through supernatural means and had then been shaped by a series of catastrophic events caused by forces which no longer prevailed.

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 1965 paper ^[1] , as:

1. Uniformity of law;
2. Uniformity of kind;
3. Uniformity of degree; and
4. Uniformity of result.

The first sense of uniformity was almost universally accepted and quickly became part of the scientific consensus; the fourth was 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 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 geological processes radically different to 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 10,000 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. 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.

Before continental drift (see plate tectonics) was recognized in the 20th century, the surface of Earth was believed to have remained generally unchanged since its formation. 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.

• Gradualism
• Catastrophism
• Scientific consensus
• Paradigm shift
• History of geology
• History of paleontology
• History of the modern theory

• Uniformitarianism discussed in great detail at PhysicalGeography.net
• Uniformitarianism discussed at the Geography section of About.com
• Uniformitarianism discussed at the Columbia Encyclopedia, sixth edition, on-line.