Statements, Quantifiers and Negation
Read mathematical statements as objects: connectives, the two quantifiers, and the order they appear in. Negating a quantified statement correctly is the skill everything else rests on.
What a learner can do afterwards
- Negate a statement with two quantifiers and read the result aloud
- Show that swapping the order of two quantifiers changes the claim
- Distinguish a conditional from its converse and its contrapositive
1 · Read
Order decides meaning. There is a teacher who praised every student picks one teacher for all students. For every student there is a teacher who praised them lets the teacher change per student. Swapping two matching quantifiers changes nothing: for every x for every y matches for every y for every x, since both cover all pairs.
A conditional says if P then Q, false only when P holds and Q fails. Its converse swaps to if Q then P and can fail: every multiple of 4 is even, yet 6 is even and no multiple of 4. Its contrapositive negates and swaps to if not Q then not P, and always matches the original. For rain cancelling a match, the contrapositive is: if the match is not cancelled, it did not rain.
Flip a double claim in order. Not for every x turns into there is an x. Not there is a y turns into for every y. Then flip the inner claim: not greater than turns into less than or equal to.
The same flips work on plain sentences. Not there is a city turns into for every city. Not every bus is late turns into there is a bus that is not late. Read the result aloud to check it sounds plain.
Flip each quantifier to negate, mind the order, and trust only the contrapositive.
The rewritten readings were written for this dataset.
2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
14 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.