From Murder on the Orient Express
Hercule Poirot: You are a philosopher, Mademoiselle.
Mary Debenham: That implies a detached attitude. I think my attitude is more selfish. I have learned to save myself useless emotion.
There is nothing so dangerous for anyone who has something to hide as conversation! Speech, so a wise old Frenchman said to me once, is an invention of man’s to prevent him from thinking. It is also an infallible means of discovering that which he wishes to hide. A human being, Hastings, cannot resist the opportunity to reveal himself and express his personality which conversation gives him. Every time he will give himself away.
From The A.B.C. Murders (Hercule Poirot #13) by Agatha Christie