What did Charles Lyell mean by using present-day processes to interpret the past?

Charles Lyell’s approach, known as uniformitarianism, posits that the physical, chemical, and biological processes observed today are the same ones that have shaped the Earth throughout its history. By using the phrase "the present is the key to the past," Lyell argued that scientists could interpret the geologic record by studying ongoing natural phenomena, such as sedimentation and weathering, rather than relying on supernatural explanations or catastrophic events.123

In his influential work, Principles of Geology, Lyell established two core propositions:
* The causes of geologic change operating today are the same as those that have acted since the earliest times.3
* These natural causes have always operated at the same average levels of energy.3

By demonstrating that Earth’s features could be explained by these consistent, observable laws, Lyell rejected the then-prevailing theory of catastrophism and provided a foundational methodology for modern geology.32