display all the ideas for this combination of texts
4 ideas
3809 | If complex logic requires rules, then so does basic logic [Searle] |
Full Idea: If you think you need a rule to infer q from 'p and (if p then q)', then you would also need a rule to infer p from p. | |
From: John Searle (Rationality in Action [2001], Ch.1.II) |
8078 | Modus ponens is one of five inference rules identified by the Stoics [Chrysippus, by Devlin] |
Full Idea: Modus ponens is just one of the five different inference rules identified by the Stoics. | |
From: report of Chrysippus (fragments/reports [c.240 BCE]) by Keith Devlin - Goodbye Descartes Ch.2 | |
A reaction: Modus ponens strikes me as being more like a definition of implication than a 'rule'. Implication is what gets you from one truth to another. All the implications of a truth must also be true. |
6023 | Every proposition is either true or false [Chrysippus, by Cicero] |
Full Idea: We hold fast to the position, defended by Chrysippus, that every proposition is either true or false. | |
From: report of Chrysippus (fragments/reports [c.240 BCE]) by M. Tullius Cicero - On Fate ('De fato') 38 | |
A reaction: I am intrigued to know exactly how you defend this claim. It may depend what you mean by a proposition. A badly expressed proposition may have indeterminate truth, quite apart from the vague, the undecidable etc. |
3810 | In real reasoning semantics gives validity, not syntax [Searle] |
Full Idea: In real-life reasoning it is the semantic content that guarantees the validity of the inference, not the syntactical rule. | |
From: John Searle (Rationality in Action [2001], Ch.1.II) |