Logic: A Practical Handbook

Points clés

• Put any family decision on one sheet, with the claim on top and reasons and evidence beneath, and mark which reasons only work together.
• Before a house rule goes on the fridge, test all four cases and choose if, only if or if and only if on purpose.
• Check a forward by asking what else you would expect to see if it were true, because missing expected evidence is evidence.
• An always or never claim falls to one counterexample, but a most or usually claim needs a count, so test each the right way.
• Name the move, never the person, and make whoever spots a bad argument offer the repaired version, adults included.

À l'intérieur de ce bit

1. The one-sheet argument map

   An argument map is a sheet of paper that shows how the reasons for a decision hang together. Write the claim in a box at the top.

2. Writing house rules without loopholes

   Children are natural lawyers, and loose if-then rules are their favourite material. If you finish your homework, you can watch the match promises the match to anyone who finishes; strictly, it says nothing about someone who has not.

3. The five-question check for forwarded messages

   Family chats carry a steady stream of urgent claims, and a forward costs nothing to share and something to believe. The check takes five questions and about three minutes.

4. The counterexample hunt for always and never

   Sweeping claims are cheap to make and cheap to test. The procedure has three steps.

5. A dinner-table field guide to six moves

   Six moves cover most of the bad arguments a family hears in a week. Attacking the person answers the speaker instead of the point.

6. Debate night and the monthly rule review

   Once a week, for twenty minutes, hold a debate night on a question that matters but does not hurt, such as whether to get a dog or whether homework comes before play. Everyone argues the side they disagree with first.