What principle states that physical processes we observe today also operated in the past?

The principle that the physical processes we observe today also operated in the past is known as uniformitarianism. This foundational concept in geology asserts that the natural laws and processes governing the Earth's surface, such as erosion, sedimentation, and volcanic activity, have remained consistent over geological time.

Uniformitarianism allows geologists to make inferences about the Earth's history by studying present-day processes. For instance, by observing how rivers shape landscapes today, scientists can infer similar processes occurred in the past to shape ancient river valleys. This principle underlies much of geological thinking and contrasts with the idea that catastrophic events or unique occurrences explain all geological features.

In the context of the other concepts, while superposition concerns the layering of rocks and helps determine relative ages, original horizontality refers specifically to sedimentary layers being deposited in horizontal layers. Stratification describes the layering itself. None of these concepts capture the broader idea of historical consistency in geological processes like uniformitarianism does.