I've been reading the Sherlock Holmes series of novels for a while now. Whether it's "A Study in Scarlet Letter" or "The Four Signatures", there is a chapter titled "The Science of Deduction" (The Science of Deduction). This aroused my curiosity - what exactly is deductive reasoning? We know there are other modes of reasoning, such as inductive reasoning and abductive reasoning, but how do they differ?

Logical reasoning, in short, is drawing conclusions from a number of observations and facts. There are three types of logical reasoning: deductive, inductive, and retrospective (abductive).

Deduction

Deduction is the process of drawing conclusions from known premises or assumptions by means of logical rules and inferences. That is deduction from universal laws to specific situations. Example:

• Major premise: All men are mortal.
• Minor premise: Jim is a man.
• Conclusion: Therefore, Jim is mortal.

Induction

Induction is the process of starting from a particular situation and summarizing the universal law through observation and induction. Example:

• Nine out of ten people say that A Study in Scarlet Letter is a good book
• Conclusion: A Study in Scarlet Letter is a good book

Abduction

Abduction reasoning is the process of working backward from results to possible causes or hypotheses. Example:

I have symptoms of diarrhea, which can have two possible causes:

A. I have eaten unclean food; B. I ate food that gave me a gastrointestinal allergy.

Observe that my co-workers ate the same food as I did today and they did not have symptoms of diarrhea, so rule out A. The barista today used a brand of milk that I had not drunk, so it could be a milk allergy, and reason B is more likely to hold true.

From the English root

Deduction, Induction, and Abduction all have the same suffix ~-duct~, which comes from the Latin ducere, meaning "to lead".

The prefix ~de-~ of deduction comes from the Latin word ~de~, meaning "down, off, away, from among, down from". So deduction literally means "lead from" which means "to reason out a conclusion from a known premise".

The prefix ~in-~ of induction comes from Latin, meaning "into, in, on, upon", so induction literally means "lead into, bring in", i.e. "to summarize a conclusion from an observed phenomenon".

The prefix ab of abduction comes from the Latin word meaning "off, away from". The word abduct can be understood as "to be led to a new path or direction away from the original path or direction", which can be derived from the meaning of "to reach a reasonable conclusion or explanation by reasoning, analyzing and studying possible situations".

Eggs

In Season 2, Episode 15 of The Big Bang Theory, Leonard is on the phone, and his lines and tone of voice, shows that it's not something pleasant. That's when Sheldon does some deductive reasoning. The result is incorrect but very funny. Let me end this post with their scripts.

Leonard (entering on the phone): I’m really very busy. Is there any way that we can put this off until I have more time to prepare? Of course. But, uh, you understand my trepidation.

Penny: What’s that about?

Howard: Not a clue.

Leonard: Can’t we just postpone it till the spring? Maybe next summer?

Sheldon: This should be fairly easy to deduce. He’s holding the phone to his left ear. Ears do not cross hemispheres, so he’s using the analytical rather than the emotional side of the brain, suggesting that he has no personal relationship with the caller.

Leonard: No, I didn’t realize it had been so long. Sure, I guess there’s no other choice but to just go ahead and do it.

Sheldon: He’s referring to an activity he has done before. It’s unpleasant and needs to be repeated. This suggests some sort of invasive medical test, like perhaps a colonoscopy.

Leonard: Aren’t there any other options? There’s not a lot of room, it’s gonna be uncomfortable.

Sheldon: Yes, yes. Yeah, I’m definitely going with colonoscopy.

Leonard: Okay, bye. My mother’s coming to visit.