In 1946 Emil Leon Post (Bulletin of Amer. Math. Soc. 52 (1946), 264 - 268) defined a famous correspondence decision problem which is nowadays called the Post Correspondence Problem, and he proved that the problem is undecidable. In this article we follow the steps of Post, and give another, simpler and more straightforward proof of the undecidability of the problem using the same source of reduction as Post original did, namely, the Post Normal Systems.