Parmenides was a Pre-Socratic Philosophers who went against the grain and whose ideas were completely oposite to everyone else's at the time.
While philosophers have started to view universe as an ever-changing system (Everything is in permanent state of flux), Parmenides proposed that this is not true, that nothing ever changes, and that reality is one big unchanging blob. We think we see the change in the universe with our senses, but Senses are deceptive and they create the illusion of change.
He came to his conclusions using deductive process that looked something like this:
- if something exists, it exists
- if something doesnt exist, it doesnt
- nothing can exist and not exist at the same time
- a point of time when a universe was in a state of nothing existing was impossible
- nothing can come into existence from a place where nothing exists
- something cannot come from nothing
- unverse must have always existed, and is eternal
His most important idea is that you have to trust the process to arrive at the conslusion - it's not enough to just take whatever "appears to your senses" as the truth.
This is a very important concept for the future of philosophy, but just like many of his peers at the time, he comes to a good conclusion and then tries to apply it to absolutely everything he sees.
Many philosophers that came after him agreed with him on many things, but couldn't swallow the idea of unchangging universe, so they tried to pick his ideas up and interpret this part of the teaching in some other way.