Every puzzle here runs on logic. 60 short lessons climb from true and false to the techniques that solve sudoku, Queens and nonograms, with practice on every page.
Each level uses the one before it. Truth and reasoning are the tools; rules as logic shows every puzzle is a handful of sentences; techniques are the deductions that solve them.
Sentences, not, and, or, if … then.
Turning what you know into what must be true.
For every, there is, exactly one.
Every puzzle’s rules are a handful of sentences.
The deductions that solve every grid on the site.
5Techniques
How to solve each puzzle on the site.
6Puzzle guides
| Symbol | Read it as | Also written |
|---|---|---|
| ∀ | For every | (x), (∀x) |
| ∃ | There is | (∃x) |
| ∃! | There is exactly one | |
| ¬ | Not | ~, − |
| ∧ | And | &, · |
| ∨ | Or (inclusive) | + |
| → | If … then | ⊃, ⇒ |
| ↔ | If and only if | ≡, ⇔ |
| ≠ | Is a different thing from | |
| ∴ | Therefore |