13 ideas
| 10282 | Logic is the study of sound argument, or of certain artificial languages (or applying the latter to the former) [Hodges,W] |
| Full Idea: A logic is a collection of closely related artificial languages, and its older meaning is the study of the rules of sound argument. The languages can be used as a framework for studying rules of argument. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.1) | |
| A reaction: [Hodges then says he will stick to the languages] The suspicion is that one might confine the subject to the artificial languages simply because it is easier, and avoids the tricky philosophical questions. That approximates to computer programming. |
| 10283 | A formula needs an 'interpretation' of its constants, and a 'valuation' of its variables [Hodges,W] |
| Full Idea: To have a truth-value, a first-order formula needs an 'interpretation' (I) of its constants, and a 'valuation' (ν) of its variables. Something in the world is attached to the constants; objects are attached to variables. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.3) |
| 10284 | There are three different standard presentations of semantics [Hodges,W] |
| Full Idea: Semantic rules can be presented in 'Tarski style', where the interpretation-plus-valuation is reduced to the same question for simpler formulas, or the 'Henkin-Hintikka style' in terms of games, or the 'Barwise-Etchemendy style' for computers. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.3) | |
| A reaction: I haven't yet got the hang of the latter two, but I note them to map the territory. |
| 10285 | I |= φ means that the formula φ is true in the interpretation I [Hodges,W] |
| Full Idea: I |= φ means that the formula φ is true in the interpretation I. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.5) | |
| A reaction: [There should be no space between the vertical and the two horizontals!] This contrasts with |-, which means 'is proved in'. That is a syntactic or proof-theoretic symbol, whereas |= is a semantic symbol (involving truth). |
| 10288 | Down Löwenheim-Skolem: if a countable language has a consistent theory, that has a countable model [Hodges,W] |
| Full Idea: Downward Löwenheim-Skolem (the weakest form): If L is a first-order language with at most countably many formulas, and T is a consistent theory in L. Then T has a model with at most countably many elements. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.10) |
| 10289 | Up Löwenheim-Skolem: if infinite models, then arbitrarily large models [Hodges,W] |
| Full Idea: Upward Löwenheim-Skolem: every first-order theory with infinite models has arbitrarily large models. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.10) |
| 10287 | If a first-order theory entails a sentence, there is a finite subset of the theory which entails it [Hodges,W] |
| Full Idea: Compactness Theorem: suppose T is a first-order theory, ψ is a first-order sentence, and T entails ψ. Then there is a finite subset U of T such that U entails ψ. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.10) | |
| A reaction: If entailment is possible, it can be done finitely. |
| 10286 | A 'set' is a mathematically well-behaved class [Hodges,W] |
| Full Idea: A 'set' is a mathematically well-behaved class. | |
| From: Wilfrid Hodges (First-Order Logic [2001], 1.6) |
| 19087 | The meaning or purport of a symbol is all the rational conduct it would lead to [Peirce] |
| Full Idea: The entire intellectual purport of any symbol consists in the total of all modes of rational conduct which, conditionally upon all the possible different circumstances and desires, would ensue upon the acceptance of the symbol. | |
| From: Charles Sanders Peirce (Issues of Pragmaticism [1905], EP ii.246), quoted by Danielle Macbeth - Pragmatism and Objective Truth p.169 n1 | |
| A reaction: Macbeth says pragmatism is founded on this theory of meaning, rather than on a theory of truth. I don't see why the causes of a symbol shouldn't be as much a part of its meaning as the consequences are. |
| 20508 | How people vote should be on public record, so they can be held accountable [Mill, by Wolff,J] |
| Full Idea: Mill believed in an open vote. People should be held accountable for how they vote, and therefore it should be a matter of public record. | |
| From: report of John Stuart Mill (Representative Government [1861]) by Jonathan Wolff - An Introduction to Political Philosophy (Rev) 3 'Representative' | |
| A reaction: Nowadays it is a mantra that voting should be secret, because coercion is an obvious problem, but MPs vote publicly, and are held accountable for their voting records. People like the mafia seem to make open public voting impossible. |
| 20507 | Voting is a strict duty, like jury service, and must only be aimed at the public good [Mill] |
| Full Idea: The citizen's vote is not a thing in which he has an option; it has no more to do with his personal wishes than the verdict of a juryman. ...he is bound to give it according to his best and most conscientious opinion of the public good. | |
| From: John Stuart Mill (Representative Government [1861], p.299), quoted by Jonathan Wolff - An Introduction to Political Philosophy (Rev) 3 'Representative' | |
| A reaction: Mill was also concerned that voters might pursue 'class interest' (which they currently do, big time). |
| 20505 | Direct democracy is inexperience judging experience, and ignorance judging knowledge [Mill] |
| Full Idea: At its best [direct democracy] is inexperience sitting in judgement on experience, ignorance on knowledge. | |
| From: John Stuart Mill (Representative Government [1861], p.232), quoted by Jonathan Wolff - An Introduction to Political Philosophy (Rev) 4 'Representative' | |
| A reaction: Recent experiments have suggested that inexperienced people can become very good at making large decisions, if they are allowed to consult experts when they want to. See Van Reybrouck's 'Against Elections'. |
| 20504 | People can only participate in decisions in small communities, so representatives are needed [Mill] |
| Full Idea: Since all cannot, in a community exceeding a single small town, participate personally in any but some very minor portions of the public business, it follows that the ideal type of a perfect government must be representative. | |
| From: John Stuart Mill (Representative Government [1861], p.217-8), quoted by Jonathan Wolff - An Introduction to Political Philosophy (Rev) 4 'Representative' | |
| A reaction: Wolff offers Mill as the principal spokesman for representative democracy. It is not only the difficulty of achieving participation, but also the slowness of decision-making. Modern technology may be changing all of this. |