Opposition of proposition — types of opposition — inference by Opposition of propositions — oppositions of singular proposition

A witness says ‘All the documents in that file are forged.’ If you can show even one genuine document was in that file, you have not shown every document is genuine — you have only shown the witness's universal claim is false. Knowing exactly what follows, and what does not, once you know one A/E/I/O proposition is true or false is the entire subject of opposition, and it is tested almost every single year.

In two minutes

Opposition is the logical relation between two categorical propositions that share the same subject and predicate terms but differ in quantity, quality, or both; the traditional Square of Opposition sets out exactly four such relations — contrary, subcontrary, subalternation and contradictory — each with its own fixed rule for what the truth or falsity of one proposition tells you about the other.

The rest of this chapter

The full explanation, the cheat sheet, every previous-year question at every mark weight, the quiz and the answer skeletons are part of Logic – I on LexBlox Semester. Sign in to unlock the subject, or read Module 1 of Legal Language and Writing free.