I mentioned before that I’m enjoying BBC’s Sherlock on Netflix. The character of Sherlock Holmes is well known for his amazing skills of observation and his “powers of deduction”. However, contrary to Holmes’ assertion that he “deducing” when he solves cases, he doesn’t actually use deductive reasoning in his process. Rather, he arrives at his conclusions through inductive reasoning, where observations of previous instances are used to form generalizations about future instances.
If you watch enough apples fall to the ground, then you may form the general hypothesis that apples always fall down, instead of maybe sideways or up. Based on your observation of apples falling, you may make the general assumption that pears, too, fall to the ground because they share similar properties with apples. And so on. This is how science progresses and, likewise, how our favorite detective solves cases.
In their book Plato and Platypus Walk Into a Bar…, authors Thomas Cathcart and Daniel Klein use the following story as an illustration:
Holmes and Watson are on a camping trip. In the middle of the night Homes wakes up and gives Dr. Watson a nudge. “Watson,” he says, “look up in the sky and tell me what you see.”
“I see millions of stars, Holmes,” says Watson.
“And what do you conclude from that, Watson?”
Watson thinks for a moment. “Well,” he says, “astronomically, it tells me that there are millions of galaxies and potentially billions of planets. Astrologically, I observe that Saturn is in Leo. Horologically, I deduce that the time is approximately a quarter past three. Meteorologically, I suspect that we will have a beautiful day tomorrow. Theologically, I see that God is all-powerful, and we are small and insignificant. Uh, what does it tell you, Holmes?”
“Watson, you idiot! Someone has stolen our tent!”
Poor Watson. In any case, the authors go on to explain how the above conclusion is arrived at through inductive reasoning:
We don’t know exactly how Holmes arrived at his conclusion, but perhaps it was something like this:
- I went to sleep in a tent, but now I can see the stars.
- My intuitive working hypothesis, based on analogies to similar experiences I have had in the past, is that someone has stolen our tent.
- In testing that hypothesis, let’s rule out alternative hypotheses:
a) Perhaps the tent is still here, but someone is projecting a picture of stars on the roof of the tent. This is unlikely, based on my past experiences of human behavior and the equipment that experience tells me would have to be present in the tent and obviously isn’t.
b) Perhaps the tent blew away. This is unlikely, as my past experiences lead me to conclude that the amount of wind would have awakened me, though perhaps not Watson.
c) Etc., etc., etc.
- No, I think my original hypothesis is probably correct. Someone has stolen our tent.