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Single Idea 12197

[from 'Introduction to Mathematical Philosophy' by Bertrand Russell, in 10. Modality / B. Possibility / 8. Conditionals / c. Truth-function conditionals ]

Full Idea

In order that it be valid to infer q from p, it is only necessary that p should be true and that the proposition 'not-p or q' should be true.

Gist of Idea

Inferring q from p only needs p to be true, and 'not-p or q' to be true

Source

Bertrand Russell (Introduction to Mathematical Philosophy [1919], XIV)

Book Reference

Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.153


A Reaction

Rumfitt points out that this approach to logical consequences is a denial of any modal aspect, such as 'logical necessity'. Russell observes that for a good inference you must know the disjunction as a whole. Could disjunction be modal?...