In philosophy, the concept of becoming originated in eastern ancient Greece with the philosopher Heraclitus of Hephesus, who in the sixth century BCE, said that nothing in this world is constant except change and becoming. His theory stands in direct contrast to Parmenides, another Greek philosopher, but from the italic Magna Grecia, who believed that the ontic changes or "becoming" we perceive with our senses is deceptive, and that there is a pure perfect and eternal being behind nature, which is the ultimate truth. In philosophy, the word "becoming" concerns a specific ontological concept which should not be confused with process philosophy as a whole or with the related study of process theology.