Uniformitarianism is the assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe. It refers to invariance in the metaphysical principles underpinning science, such as the constancy of causal structure throughout space-time, but has also been used to describe spatiotemporal invariance of physical laws. Though an unprovable postulate that cannot be verified using the scientific method, uniformitarianism has been a key first principle of virtually all fields of science. In geology, uniformitarianism has included the gradualistic concept that "the present is the key to the past" and is functioning at the same rates, though many modern geologists no longer hold to a strict gradualism. Coined by William Whewell, it was originally proposed in contrast to catastrophism by British naturalists in the late 18th century, starting with the work of the Scottish geologist James Hutton. Hutton's work was later refined by John Playfair and popularised by Charles Lyell's Principles of Geology in 1830.

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