The Essential Guide to Logic

Key takeaways

• Logic fits on one page: arguments, three ways of supporting them, the ways reasoning fails, and the goal of matching belief to evidence.
• Before judging a claim, ask whether it rests on deduction, induction or explanation, because each has its own test and its own way of failing.
• Deduction is certain but adds nothing new, while induction and explanation add something new but can be wrong, so hold their conclusions provisionally.
• Catch fallacies by checking the argument and counter biases by changing the process, such as keeping a written log instead of trusting memory.
• Start with four habits, find the claim, surface the assumption, apply the right test and match your confidence, and leave fallacy lists for later.

Inside this bit

1. The whole map on one page

   Logic can look like a sprawling subject full of symbols and Latin names, but its core fits on one page. There are six big ideas.

2. Idea one: a claim plus its reasons

   An argument, in logic, is not a quarrel. It is a claim, called the conclusion, supported by one or more reasons, called premises.

3. Deduction: certainty that comes from form

   Deduction is reasoning in which true premises guarantee the conclusion. The guarantee comes from the shape of the argument, not its subject.

4. Induction and explanation: reasoning without guarantees

   Most real decisions rest on reasoning that cannot guarantee its conclusion. Induction moves from observed cases to a general pattern or a prediction, and its strength depends on how many cases you saw and how varied they were.

5. Fallacies and biases: how reasoning fails

   A fallacy is a flaw in an argument, a pattern that looks like good reasoning but is not. Four turn up constantly.

6. What to prioritise and what to ignore

   Beginners often start in the wrong place. Long lists of fallacy names, symbols such as arrows and upside-down letters, and the art of winning debates are all real parts of logic, and all of them can wait.