Logical rules for negating complex statements; useful for clear thinking about opposites.
De Morgan's 1847 laws: not (A and B) equals (not A) or (not B). Their practical use is in specification and negotiation, where negating a compound condition is where most reasoning errors occur.
In practice
- A contract clause requiring "not late and not incomplete" is satisfied by being either on time or complete, unless the negation is worked through.
- A policy prohibiting staff who are "not qualified and not supervised" permits the unqualified if supervised, which is what the drafter meant and not what most readers take.
- Further reading
- Wikipedia
- Subject
- Decision-making & Reasoning
- Essay
- The catalogue of predictable errors