Parmenides

Unlike the Milesian philosophers ([Thales](thalesofMiletus),
[Anaximander](anaximander), [Anaximenes](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.

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References:

1.  Sophie's World by Jostein Gaarder