“Never tell all you know—not even to the person you know best.”

“But I know human nature, my friend, and I tell you that, suddenly confronted with the possibility of being tried for murder, the most innocent person will lose his head and do the most absurd things.”

“You tell your lies and you think nobody knows. But there are two people who know. Yes- two people. One is le bon Dieu - and the other is Hercule Poirot”

“You surprise me, Hastings. Do you not know that all celebrated detectives have brothers who would be even more celebrated than they are were it not for constitutional indolence?”

“How little you might know of a person after living in the same house with them!”

“You start into it, inflamed by an idea, full of hope, full indeed of confidence. If you are properly modest, you will never write it at all, so there has to be one delicious moment when you have thought of something, know just how you are going to write it, rush for a pencil, and start buoyed up with exaltation. You then get into difficulties, don’t see your way out, and finally manage to accomplish more or less what you first meant to accomplish, though losing confidence all the time. Having finished it, you know it is absolutely rotten. A couple of months later, you wonder if it may not be all right after all.”

“Everybody always knows something," said Adam, "even if it's something they don't know they know.”