Reason and logic are detective Hercule Poirot's weapons of choice. Poirot relies upon his insight into human nature, his powers of observation, and the deductive method in order to get to the truth of things. As you will notice, he is always careful in advancing his points and never makes false assumptions. In this way, the novel offers us an example of incredibly sound judgment. Poirot, after all, does unravel the complicated plot in the end. We might also say that though the novel is about a murder, it is optimistic in that it has great faith in the power of orderly reason to resolve complicated problems – and to see through to the truth of things.
Reason and logic don't come naturally to any of the characters in the novel, except for Poirot.
While we can think our way through some things, life is not lived solely in the head.