Complex sentence: They did not, like Heraclitus, consider the coming into existence and motion as the existence of things, and rest and tranquillity as the non-existence, because they had derived from the Eleatics the conviction that an existence could not pass into non-existence, and vice versa.
Split into simpler sentences: They did not, like Heraclitus, consider coming into existence and motion as the existence of things, and rest and tranquillity as the non-existence. This is because they had derived from the Eleatics the conviction that an existence could not pass into non-existence, and vice versa.