Uniformitarianism is the idea that the same physical laws of today have always operated. It was the centerpiece of James Hutton's 1795 geology book Theory of the Earth, with proofs and illustrations.

In this work Hutton proposed that the causes acting on the world today also acted in the past. This means that the rocks of the Earth were formed by processes which we can see operating today. These processes, though very slow (gradualism), made the world what it is today. The idea is part of a wider philosophy called the philosophy of naturalism.