What I learned from reading Parmenides' (540 BC) fragments, which were preserved in Simplicius's commentary:
The old argument was whether the world is composed of small parts that make up the whole or whether the whole is just one big, unified whole. One of the thinkers who believed that the universe and the world are one big unchanging whole was Parmenides. This belief had implications for justice and how we perceive meaning in our life experience.
According to Parmenides, truth ("what it is") is one, continuous, and has no beginning or end. He argued that the whole has no beginning, reasoning that "if it came from nothing, what need could have made it arise later rather than sooner?" Therefore, he encouraged us to seek knowledge by focusing on the whole rather than on fragments, appearances, and human-named objects.
First, he was wrong in claiming that we cannot learn from negations ('what it is not'). Actually, we have learned a great deal by modeling our knowledge probabilistically, using information we gathered from what does not exist.
Secondly, He argued that the big whole "remains constant in its place; for hard Necessity keeps it in the bonds of the limit that holds it fast on every side." Here he might have been wrong here partially because, in physics, we have identified that the universe is expanding. This is shown by the light we receive on Earth, which is redshifted (its wavelength is longer than it would be if the planets and stars were stationary).
He is partially correct because however, he is saying, I think, that there is an underlying universal physical law. In the end, I think we can remember that the law—the divine, as Parmenides calls it—is constant. Quantum mechanics, which is probabilistic in nature, helps us model the known "what is." In the end, atomic theory and quantum mechanics are tools for describing and understanding the universe, but the fact remains that life is deterministic (even the knowledge from modeling the indeterministic microscopic pieces in quantum mechanics). Whether the universe is expanding or staying the same is a fact. It is binary. The whole and truth is constant.
Parmenides might have been wrong in suggesting that we should seek knowledge only by thinking about what is real, rather than considering negation and what does not exist. But he might have been right in his final that there is a law that unites everything and that we are part of the whole. The most life-affirming aspect of his philosophy was his belief that we can learn about "what is" and the whole from anything we encounter. In the end, he suggested, learning one thing leads to learning them all.