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Only Two Kinds of Reasoning

June 14, 2021

We humans can think logically in only two ways: deductively and inductively.

Deduction is great when we can manage it! The problem is, life doesn’t offer the sort of evidence demanded by deductive reasoning very often. This is why mathematics has been called the only true science. Everything in math is true if the basic assumptions of math are true.

Mathematicians sometimes engage in induction as they search for answers, but eventually, they must end of up with deduction.

One way to understand deduction is to draw a line. Below the line, place a claim. Above the line place reasons to believe the claim. If there is enough information to know that the claim must be true, then the argument is deductive.

A deductive argument:

All men are mortal.

Socrates was a man.

Socrates was mortal.

If I don’t have enough information to be sure of my claim, then I have an inductive argument.

An inductive argument:

John is a hardworking, intelligent person.

John wants to work for me.

I should hire John to work for me.

You can see that the first argument has all the evidence needed to conclude that Socrates was mortal. But you can also see that I don’t have enough information to hire John. 

Sometimes evidence is strong, but not strong enough to be sure of a  claim.

In 2004, a small plane crashed in Glacier National Park. It was winter, and as rescuers flew over the site, they discovered some discouraging facts. (1) The plane was totally destroyed by fire (2) there were no tracks in the snow around the crash site (3) the temperature fell to 20 degrees overnight. The rescuers concluded that there were no survivors; however, the next day, two people were found walking out of the area. They were the pilot and passenger of the aircraft.

Obviously, the evidence of their death was not conclusive.

So you can see that in thinking we can go back and forth between induction and deduction, but spend most of our time in induction.

The great weakness of induction is that it is a prediction of the future. The great British philosopher Bertrand Russell makes the co-called “chicken” analogy. A chicken wakes up everyday believing everything will be fine (a prediction because everyday so far in her life everything has been fine).

But one day, the chicken gets up and it’s her last day! The farmer chops off her head and has fried chicken for supper.

Do you believe the following claim? “The sun will come up tomorrow.” I certainly do, but the problem is that the claim is an example of inductive thinking—it’s only a prediction. I can’t know absolutely that the sun will come up tomorrow.

Fortunately, our lives do not depend upon absolute knowledge at all times.