My philosophy of teaching

Types of reasoning

Inductive vs. deductive reasoning

What is Sherlock Holmes known for? For amazing leaps of deduction, you might say. The Wikipedia entry on Holmesian deduction provides a helpful quote from “A Scandal in Bohemia”, in which Holmes tells Watson that he had gotten very wet lately and that he had “a most clumsy and careless servant girl”. When Watson demands to know how Holmes could have made such a detailed and accurate guess, Holmes explains:

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.

You may have noticed that I characterize Holmes’ ratiocination as guessing, but he himself – as well as Wikipedia – calls it deduction. I was trying to be polite, as you can surmise by trying to categorize the example in the terms of the table of Two types of reasoning:

Table 7 Two types of reasoning
  induction deduction
premise 1 Sherlock is a grandfather. All men are mortal.
premise 2 Sherlock is bald. Sherlock is a man.
conclusion All grandfathers are bald. Sherlock is mortal
characterization specific > general general > specific
process bottom-up top-down

In the example, Holmes starts with a very specific observation and works backwards to a general cause. Yet Two types of reasoning classifies this as induction, not deduction. So to the extent that the example is representative of Holmesian reasoning, it is almost exclusively inductive. Yet you can appreciate why Holmes would want to say that it is deductive. If the premises are true in deduction, then the conclusion is guaranteed to be true, too. Not so in induction: even true premises can lead to a false conclusion.

The next topic

Come to class having read The fields of linguistics and answered the questions.

Last edited: Aug 26, 2019