Who said "panta rhei" and what did he mean by this saying?

Philosopher Heraclitus, is the right answer.

Explanation:

Heraclitus of Ephesus was a was a philosopher from Greek. He is best known for the doctrine he gave to the world about the center of the universe. According to him, Change is central to the universe. In this context, he gave the phrase "panta rhei" which implies "everything flows".  

This popular saying used to describe Heraclitus' idea grows from Simplicius, a Neoplatonist, and Cratylus of Plato.

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