“I reckon you’ll be shocked,” replied Greywood Usher, “as I know you don’t cotton to the march of science in these matters. I am given a good deal of discretion here, and perhaps take a little more than I’m given; and I thought it was an excellent opportunity to test that Psychometric Machine I told you about. Now, in my opinion, that machine can’t lie.”
“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—if 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.”
G. K. Chesterton, "The Mistake of the Machine", The Wisdom of Father Brown. A point that many people these days need to remember.
