"But then, how do you know?"
"Because I am Hercule Poirot I do not need to be told."

"I saw a particular personage and I threatened him — yes, Mademoiselle, I, Hercule Poirot, threatened him." "With the police?" "No," said Poirot drily, "With the Press — a much more deadly weapon."

Whenever a man talks he lies, and so far as he talks to himself — that is to say, so far as he thinks, knowing that he thinks — he lies to himself. The only truth in human life is that which is physiological. Speech — this thing that they call a social product — was made for lying.

Mademoiselle, I beseech you, do not do what you are doing.” “Leave dear Linnet alone, you mean!” “It is deeper than that. Do not open your heart to evil.” Her lips fell apart; a look of bewilderment came into her eyes. Poirot went on gravely: “Because — if you do — evil will come…Yes, very surely evil will come…It will enter in and make its home within you, and after a little while it will no longer be possible to drive it out.” Jacqueline stared at him. Her glance seemed to waver, to flicker uncertainly. She said: “I — don’t know — ” Then she cried out definitely, “You can’t stop me.” “No,” said Hercule Poirot. “I cannot stop you.” His voice was sad.

"A man may say, "From now on I'm going to speak the truth." But the truth hears him and runs away and hides before he's even done speaking."

You lie—under a mistake, For this is the most civil sort of lie That can be given to a man's face. I now Say what I think.