Parmenides was most famous for the following statements:

What is, is.
What is not, is not.

Although these statements appear simple, in fact they make a profound statement on the existence in the material plane. What is not, is not basically says there can be no vacuum, that the concept of void is a logical error. The substance that is the One fills all the Universe. Classical Physics would deny Parmenides claim, however, modern physics has found that the quantum vacuum in fact fills the Universe just as Parmenides predicted over 2000 years ago.

Excerpt from the Encyclopedia Britannica without permission.