What I have above is a message in cipher which even beginners can solve because there are so many clues. One they are likely to notice right away is furnished by the first two letters: SB. It should be easy to see there are a number of ways of interpreting these two letters. For example, we could think of SB as standing for SH, or TH, or AR. However, once we start making a few trials in deciphering the rest of this message we will soon discover that only one of these possibilities will work. Interestingly enough, this will happen long before we finish deciphering the whole message! This is to say, long before the end the possible interpretations of this particular clue will have been narrowed down to one.
Now this is a common phenomenon, as readers can attest from their own experience. In an investigation we do not have to wait till the end to have everything pegged down. As the investigation proceeds, it will become clearer and clearer what the earlier clues should mean. There is a simple reason for this. Our interpretation of any clue has to jibe with our interpretation of the other clues. If we start off the investigation with a wrong interpretation, after a few steps things will start looking awry because the wrong interpretation will not agree with the other clues, at which point we will have to backtrack and try a different interpretation.
If the structure we are trying to reconstruct is a horse we cannot start by giving it a crocodile's head.
In our example above SB at the beginning has many possible interpretations. However, if we make the effort to try to decipher this message, we will find that after a few steps the number of possible interpretations will be narrowed down to one. This is done through trial and error, as we have said. So here we have an example of the narrowing-down process which is common in investigations. In this case the narrowing down is achieved through trial and error.
But this is not the only way in which we narrow down. In an earlier post we mentioned Watson's trip to the post-office. Without Watson telling him Sherlock Holmes was able to determine that Watson went there to send a telegram. Understandably, Watson was surprised.
“How, then, did you deduce the telegram?”
“Why, of course I knew that you had not written a letter, since I sat opposite to you all morning. I see also in your open desk there that you have a sheet of stamps and a thick bundle of postcards. What could you go into the post-office for, then, but to send a wire? Eliminate all other factors, and the one which remains must be the truth.”
--The Sign of Four
In popular parlance what Sherlock Holmes is doing here is engage in a process of elimination. There are only three things Watson could do at the post-office: buy stamps, buy postcards, or send telegram. Sherlock Holmes narrows these three things down to one: Watson went to the post-office to send a telegram.
The process of elimination is a well-known technique among all those who follow clues but it is different from the SB example we explained earlier. In the SB example we land on the truth through trial and error: the true interpretation will fit in with the other clues and by doing so enable us to advance the investigation. In a process of elimination we have no direct support for the true answer; the only thing that tells us it is true is that all the other answers are false.
Two remarks I want to enter before we close, one for each kind of narrowing down.
When we plan on using trial and error to narrow down, sometimes we can be lucky enough to have picked up the right candidate at the very first trial so that there is no need to test the others. In this case there is only one trial and no error. Inasmuch as we would congratulate ourselves when this happens, I think I can assume it does not happen all that often.
Now the other remark, this one in connection with the process of elimination .... In employing this process we first list all the possibilities, then eliminate them one by one in the hope that only one will remain. When this goes as expected we have no complaints. But sometimes there are surprises. Sometimes we manage to eliminate all the possibilities!
We shall have more to say about both ways of narrowing down by and by.
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