Proof of Ramsey's theorem

From a channel Lumen includes in full: Mathemaniac makes deep, animated explainers of university maths (complex analysis, group and Lie theory, differential forms, vector calculus) that motivate and prove the main results.

A 5-minute video from Mathemaniac's catalogue; the title names its topic.

Video by Mathemaniac, played through YouTube's embedded player.