"No machine can lie," said Father Brown, "nor can it tell the truth."
"It did in this case, as I'll show you," went on Usher positively. "I sat the man in the ill-fitting clothes in a comfortable chair, and simply wrote words on a blackboard; and the machine simply recorded the variations of his pulse; and I simply observed his manner. The trick is to introduce some word connected with the supposed crime in a list of words connected with something quite different, yet a list in which it occurs quite naturally. Thus I wrote 'heron' and 'eagle' and 'owl,' and when I wrote 'falcon' he was tremendously agitated; and when I began to make an 'r' at the end of the word, that machine just bounded. Who else in this republic has any reason to jump at the name of a newly arrived Englishman like Falconroy except the man who's shot him? Isn't that better evidence than a lot of gabble from witnesses: the evidence of a reliable machine?"
"You always forget," observed his companion, "that the reliable machine always has to be worked by an unreliable machine."
"Why, what do you mean?" asked the detective.
"I mean Man," said Father Brown, "the most unreliable machine I know of. I don't want to be rude; and I don't think you will consider Man to be an offensive or inaccurate description of yourself. You say you observed his manner; but how do you know you observed it right? You say the words have to come in a natural way; but how do you know that you did it naturally? How do you know, if you come to that, that he did not observe your manner? Who is to prove that you were not tremendously agitated? There was no machine tied on to your pulse."
"I tell you," cried the American in the utmost excitement, "I was as cool as a cucumber."
"Criminals also can be as cool as cucumbers," said Brown with a smile. "And almost as cool as you."
"Well, this one wasn't," said Usher, throwing the papers about. "Oh, you make me tired!"
"I'm sorry," said the other. "I only point out what seems a reasonable possibility. If you could tell by his manner when the word that might hang him had come, why shouldn't he tell from your manner that the word that might hang him was coming? I should ask for more than words myself before I hanged anybody."
-The Wisdom of Father Brown (1914)