Deductive Reasoning is Overrated

Modern culture worships at the altar of deductive reasoning. We have been taught that if premise 1 and premise 2 are both true, the deductive (syllogostic) structure of “if/then” will lead us to a third, irrefutable truth.

This is how that often goes wrong:

Premise 1: All birds lay eggs.
Premise 2: Snakes lay eggs.
Therefore: Snakes are birds.

We know that snakes are not birds, so we examine Premise 1 and Premise 2 to see which one is false.

Neither of them is false.

Ohhhhhh… now I see the problem. Even though “All birds lay eggs,” they are not the ONLY animal that lays eggs.

Our premise wasn’t false; it was incomplete.

It is easy to find a premise that is true,
but it is hard to find a premise that is complete.

If we did not already know that snakes aren’t birds, we would likely have embraced the conclusion.

The second problem with our birds and snakes example is that we did not begin with a larger premise and move to a smaller one. Deductive reasoning is – by definition – subtractive. Although our birds and snakes example followed the same if/then structure of deductive reasoning, it was actually inductive, which is addition rather than subtraction.

Inductive reasoning is often correct, but not always.

IF you believe that ‘if/then’ always leads to the truth, THEN you are mistaken.

(Do you see what I did there?)

– Indy Beagle