Final answer:
Uniformitarianism is the principle described by the phrase "the present is the key to the past". It states that the same natural laws and processes that operate now have always operated in the past.
Step-by-step explanation:
The principle that is described by the phrase "the present is the key to the past" is uniformitarianism. Uniformitarianism is the concept that the same natural laws and processes that we observe today have always operated in the past and apply everywhere in the universe. It suggests that geological processes have been happening at a constant rate throughout Earth's history.