4 Expressing Generality
We now turn from the logic of sentential connectives to a more comprehensive system allowing us to analyse logical inferences which turn on the internal structure of atoms and on the behaviour of locutions like 'all' and 'some' which express generality. Consider the following for example:
All logicians are rational
Some philosophers are not rational
Therefore not all philosophers are logicians.
Plainly this is valid in virtue of its form, but to demonstrate and explain its validity we need a richer language and more formal machinery than are available in the logic we have studied so far
- First order language Naming things, describing things and generalising
- Formalisation Complex sentences using quantifiers; bad news for goats!
- Quantifiers in Proofs Natural deduction rules ∀I, ∀E, ∃I, ∃E
- Examples Proofs using the rules for quantifiers
- Properties of relations Defining some important classes of binary relations
- Functions Function symbols, terms and their logic
- Examples Proofs involving relations and function symbols
- Extra (math) Other views of relations: relation algebra and graph theory
Quantifier proof exercises Sample problems with solutions