Final answer:
The principle stating that the same geologic processes active in the past are operating today is known as uniformitarianism, an idea central to modern geology.
Step-by-step explanation:
The principle that states that the same geologic forces that were active in the past are operating today is known as uniformitarianism. This concept, which is often summarized as "The present is the key to the past," was popularized by the work of Charles Lyell, a prominent figure in the field of geology during the 19th century. Uniformitarianism emphasizes that the geologic processes shaping the Earth's surface in the present, such as erosion and sedimentation, have operated in the same manner throughout Earth's history. This view contrasts with catastrophism, which posits that Earth's features formed through sudden, short-lived, violent events, rather than gradual, long-term processes.