Many people use the words “logic” or “logical” as a mark of their approval. They say an idea is logical because they believe it.

But logic is not a shortcut. Calling an idea logical does not make it true. Logic is also not a superpower. Anyone who declares, mysteriously, “Just believe me!” or “Trust me about this!” isn’t being logical and doesn’t deserve trust.

Logic is a formal thought process — a set of rules for how to think. An idea is logical if it passes the tests of the logical process. Even then, logic is a very limited aid to knowing. This short essay describes the very limited usefulness of formal logic to epistemology and to being a wise citizen.

Logic in its most basic form takes a three-line form called a syllogism. The first two lines (each called a “premise”) are assumed. The logical process requires us to accept, for the moment, that the premises are true. The third line is a conclusion. Here’s a logical syllogism:

• Premise 1: Chocolate is poison

• Premise 2: Poison is bad

• Conclusion: Chocolate is bad

The conclusion here is logical. It is also logic if you switch the premises:

• Premise 1: Poison is bad

• Premise 2: Chocolate is poison

• Conclusion: Chocolate is bad

You may object that the premise “Chocolate is poison” is not true. That objection is correct. Chocolate is not poison. But logic doesn’t require truthfulness. It only requires that the conclusion follows from the premises. Correct logic often leads to false conclusions. A conclusion can even be logical when both premises are false:

• Premise 1: Chocolate is poison

• Premise 2: Poison is good

• Conclusion: Chocolate is good

The ancient Greek philosopher Aristotle is credited with discovering or developing the first system of formal logic. One of the features of Aristotle’s system was the square of opposition, shown here. “S” stands for the subject of the premise and “P” stands for the predicate (second part) of each premise.

Any statement is true either always, or sometimes, or never. Aristotle’s method leads to the right conclusion in every instance. But only for very dumbed-down kinds of statements. If you say, for example, that “All chocolate is brown” Aristotle’s square insists (following the diagonal “contradictory” line) that “Some chocolate is not brown” cannot also be true. If you say “No murderers are men” Aristotle shows that “Some murderers are men” cannot also be true. Duh!

Life doesn’t present clear, simple syllogisms like these very often. Life is much more complicated. It presents many facts and assertions, some of them proven and others only suspected, some that contradict others and often the most essential facts are unknowable. Real-life complexity makes thinking hard, but at least makes it interesting. Life would be seriously tedious if every decision and every investigation were a three-line syllogism. Imagine how boring detective novels would be if they never got deeper than:

• Premise 1: All murderers are guilty

• Premise 2: Colonel Mustard is guilty

• Conclusion: Colonel Mustard is a murderer

Logic is an abstract intellectual discipline that has occupied many great minds through history. It can make an interesting hobby, or even a life-time pursuit. No disrespect for Aristotle, or Descartes, or Charles Dodgson or Monty Python’s Sir Bedevere is intended.

But citizens living in a complex society need much more than abstract logic. They need to seek out many sources of information. They need to carefully consider what is being said by the source — whether an idea is being declared plainly, or implied, or not actually hinted at.

Not many sources of information require the readers or viewer to use logic. Most sources force-feed the conclusion to the audience. So it is much more important to be critical and careful about what you believe than it is to apply the logical form to your thinking.

Think:

• How good are you at the game of Clue? That is a very good test of your logical abilities.

• How is a raven like a writing desk? [1]

[1] Because there is an “O” in both and an “N” in neither.