What Logic Actually Is (And What It Isn't)
An excerpt from Logic in the Wild — my new book on formal logic for people who think for a living
Most people who want to “learn logic” want one thing: to be better at winning arguments. They’ve been in conversations where they knew — knew — they were right, but couldn’t explain why. Someone made a point that felt wrong but they couldn’t identify the flaw. They got talked into something they later regretted. They want tools to fight back.
That’s a reasonable thing to want. This book will help you do exactly that. But winning arguments is not what logic is for — and if you start there, you’ll misuse every tool in the kit.
Here’s what logic is actually for: knowing what follows from what.
That sounds anticlimactic. It isn’t.
When I taught formal logic at the community college level, I started every semester the same way. I’d walk in on the first day, write a single argument on the board, and ask the class to evaluate it. The argument was this:
All dogs are mammals. All mammals are warm-blooded. Therefore, all dogs are warm-blooded.
Every hand would go up. “Valid.” “Correct.” “Obviously true.” And they were right — but for the wrong reasons. Most of them approved of the argument because they agreed with the conclusion. Dogs are warm-blooded. The conclusion is true, so the argument must be good.
Then I’d write this one:
All dogs are fish. All fish are warm-blooded. Therefore, all dogs are warm-blooded.
False premise, false premise — and the exact same conclusion. Dogs are warm-blooded. And the logical structure — the form of the argument — is identical to the first one.
The class would get uncomfortable. “That’s a bad argument,” someone would say. “Why?” I’d ask. “Because the premises are wrong.” “Right. But is the reasoning wrong?”
That pause — that moment of genuine uncertainty — is where logic education actually begins.
The distinction you need is between validity and truth. They are not the same thing. Confusing them is the source of more bad reasoning than almost anything else.
A valid argument is one where, if the premises were true, the conclusion would have to be true. The logical structure holds. The gears mesh. Validity is a structural property — it says nothing about whether the premises actually are true. That’s a separate question entirely.
A true statement is one that corresponds to reality. “Dogs are mammals” is true. “Dogs are fish” is false. Truth is a property of individual statements, not of the reasoning that connects them.
Both dog arguments above are valid. The reasoning is identical — they share the same logical form. But only the first is sound. Soundness means valid and the premises are actually true. The second argument is valid but unsound, because “all dogs are fish” is false and “all fish are warm-blooded” is false. The conclusion still happens to be true — but it got there by accident, not by reasoning.
This is an excerpt from Chapter 1 of Logic in the Wild: From Formal Proofs to Real-World Decisions. The book covers everything you’d get in a college-level introductory logic course — with real-world applications in every chapter. Ebook and workbook at ratio.press. Hardcover and paperback on Amazon.
Want a preview of what’s ahead? Grab the free Fallacy Field Guide — 35 logical fallacies on one reference card.