Examples and counterexamples
Learn what a claim covers, explain why it works, and recognise an allowed case that shows it needs changing.
Key knowledge
A claim that P(x) holds for every x in a specified domain is false if an element of that domain makes P(x) false. Testing some successful examples does not establish the claim for an infinite domain.
Values and equations
¬(∀x ∈ D, P(x)) ⇔ ∃x ∈ D, ¬P(x)
- D
- Domain of the claim
- P(x)
- Predicate asserted of x
Conditions and scope
- The counterexample must satisfy every hypothesis and belong to the stated domain.
- This concerns universal mathematical claims, not a statistical claim about typical outcomes.
Sources · 7
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Universal-claim refutation and the domain requirement.
2026-10-10 · Checked the mathematical counterexample rule; excluded the source's informal generalisations about scientific induction.
Foundational review
2026-10-10 · Fresh primary-source inspection of the complete compact record, including every statement, equation, quantity and material condition, followed by independent AI editorial review. This check does not certify all linked lessons or every passage of every cited publication.
Sources from other places
Scope or convention differs
A counterexample refutes an all-members claim in the stated set.
The ostrich example illustrates refutation; it does not independently prove every formal quantifier qualification.
Australian Mathematical Sciences Institute and University of Melbourne
- TIMES · Sets and Venn diagrams AustraliaEnglish
Subsets and the words all and if...then
- TIMES · Sets and Venn diagrams
Related articles
Examples, explanations and counterexamples ↗Entry details
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