Unlike the Milesian philosophers (Thales, Anaximander, Anaximenes) who believed that the world is made of a fundamental substance, Permenides focused on the question, "How one substance changes into another?".

It is noteworthy that at his time, the common Greek beliefs were that "nothing can come out of nothing" or "nothing that exists can become nothing". It seems very similar to energy conservation principle.

References:

1. Sophie's World by Jostein Gaarder