Classic Logic & Mini Logic
Words such as “not,” “neither,” and “didn’t” usually create X marks that can force later matches.
Interactive lesson
Using Negative Clues
Turn what is not true into a useful limit
A negative clue may not tell you the answer by itself. It tells you where the answer cannot be, and that X can become decisive when you combine it with other facts.
Try it yourself
The Autumn Bake-Off
Ada, Ben, and Cora each entered a different baked good: apple pie, berry tart, or pumpkin bread.
Every clue tells you what did not happen. Use those limits to discover what did.
1 Ben did not bring the pumpkin bread.
2 Ada did not bring the pumpkin bread.
3 Ada did not bring the berry tart.
Who brought each baked good?
| Baker | Apple Pie | Berry Tart | Pumpkin Bread |
|---|---|---|---|
| Ada | |||
| Ben | |||
| Cora |
× impossible ✓ match
Negative clue 1 of 3
Clue 1 says Ben did not bring the pumpkin bread. What can we mark with certainty?
Choose only what the highlighted clue guarantees.
The key takeaway
“Not” is information.
Clue 1 did not identify Ben’s baked good. It gave you one reliable X. Clue 2 added a second X in the same column, which forced Cora to match the pumpkin bread.
AdaApple pie
BenBerry tart
CoraPumpkin bread
When you read a negative clue, ask:
- Which exact pairing does this rule out?
- Does that X leave only one possibility in its row or column?
Do not stretch a negative clue beyond what it says. Mark the certain X first, then look for what the remaining open cells force to be true.
Use the same thinking
Negative clues narrow every kind of deduction
Sherlogic
A witness statement may rule out a person, place, or time even when it does not name the culprit.
Futoshiki
An inequality tells you which values cannot fit, and the remaining legal values reveal the answer.
Mastermind
Feedback can prove that a color or position is wrong, shrinking the set of possible codes.