Karl R. Popper

This is the first part of Chapter 7 of Karl Popper’s last book, Parmenides' World(1998). Its central theme is the invariance principle, which Popper discusses in the light of Parmenides’ idea that there should be one basic entity (or a few forms thereof) underlying the great diversity of events. Based on this idea, argues Popper, Parmenides was the first thinker to postulate the existence of the theoretical world as a reality different from (“hidden beneath”) the world of events. The essay traces the development of the invariance principle and related concepts from Parmenides to present-day science.