Within scientific philosophy, uniformitarianism is the principle in which one assumes that the same processes that shaped the Universe occurred then as they do now, unless there is good evidence otherwise. Or in other words, certain natural laws are unchangeable. Although most synonymous with geology, other sciences on a similar timescale also employ this principle, such as astronomy or evolutionary biology.

Geology

Uniformitarianism is one of the most basic principles of modern geology, the observation that fundamentally the same geological processes that operated in the distant past also operate today. 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.

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

In the 18th and 19th centuries, the debate between the two theories was very controversial, since uniformitarianism seemed to be incompatible with the prevailing religious beliefs of the time. Today, however, all mainstream scientists support uniformitarianism.

The knowledge 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 explosive volcanism occur today as they have in the past and these events provide punctuations in an over-all gradual process.