The Many Cases of Disappearing Clues

How does Sherlock Holmes know? He knows by following clues. How does he know his answers to the clues are right? He knows they are right when clues lead to new clues.

There is something interesting to notice here. Clues allow Sherlock Holmes to discover and at the same time to evaluate results.

Galileo understood this point.

 ‘All truths are easy to understand once they are discovered; the point is to discover them.’

--Galileo Galilei

The dual role that clues play in investigations is often overlooked. The common misconception is, clues allow us to discover; evaluation (of results) is separate. Nay, very often the roles clues play are forgotten altogether; the only thing left is a simple explanation of why the results are correct.

Sherlock Holmes is not beyond using this simple trick to impress Watson. In A Scandal in Bohemia he was able to tell Watson--without Watson first telling him--that Watson had been caught in the rain recently and that he had hired a servant girl who was not all that good at her job. Asked how he knew, he said,

 'It is simplicity itself ... my eyes tell me that on the inside of your left shoe, just where the firelight strikes it, the leather is scored by six almost parallel cuts. Obviously they have been caused by someone who has very carelessly scraped round the edges of the sole in order to remove crusted mud from it. Hence, you see, my double deduction that you had been out in vile weather, and that you had a particularly malignant boot-slitting specimen of the London slavey.'

The marks on Watson's shoe--who else but a Sherlock Holmes would notice?