The article explores the concept of proof, focusing on its informal side. It argues that proof is not just about convincing arguments, but rather a connection of ideas that can exist in some worlds and not in others. The idea of proof has roots in ancient philosophy, particularly in Pythagorean thought. The article also discusses the time gap between the appearance of proof and its formal definition in modern logic. Overall, the article emphasizes the importance of understanding the idea of proof beyond just convincing arguments.