25 ideas
| 10476 | The idea that groups of concepts could be 'implicitly defined' was abandoned [Hodges,W] |
| Full Idea: Late nineteenth century mathematicians said that, although plus, minus and 0 could not be precisely defined, they could be partially 'implicitly defined' as a group. This nonsense was rejected by Frege and others, as expressed in Russell 1903. | |
| From: Wilfrid Hodges (Model Theory [2005], 2) | |
| A reaction: [compressed] This is helpful in understanding what is going on in Frege's 'Grundlagen'. I won't challenge Hodges's claim that such definitions are nonsense, but there is a case for understanding groups of concepts together. |
| 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. |
| 10478 | Since first-order languages are complete, |= and |- have the same meaning [Hodges,W] |
| Full Idea: In first-order languages the completeness theorem tells us that T |= φ holds if and only if there is a proof of φ from T (T |- φ). Since the two symbols express the same relationship, theorist often just use |- (but only for first-order!). | |
| From: Wilfrid Hodges (Model Theory [2005], 3) | |
| A reaction: [actually no spaces in the symbols] If you are going to study this kind of theory of logic, the first thing you need to do is sort out these symbols, which isn't easy! |
| 10477 | |= in model-theory means 'logical consequence' - it holds in all models [Hodges,W] |
| Full Idea: If every structure which is a model of a set of sentences T is also a model of one of its sentences φ, then this is known as the model-theoretic consequence relation, and is written T |= φ. Not to be confused with |= meaning 'satisfies'. | |
| From: Wilfrid Hodges (Model Theory [2005], 3) | |
| A reaction: See also Idea 10474, which gives the other meaning of |=, as 'satisfies'. The symbol is ALSO used in propositional logical, to mean 'tautologically implies'! Sort your act out, logicians. |
| 7760 | Russell only uses descriptions attributively, and Strawson only referentially [Donnellan, by Lycan] |
| Full Idea: Donnellan objects that Russell's theory of definite descriptions overlooks the referential use (Russell writes as if all descriptions are used attributively), and that Strawson assumes they are all used referentially, to draw attention to things. | |
| From: report of Keith Donnellan (Reference and Definite Descriptions [1966]) by William Lycan - Philosophy of Language Ch.1 | |
| A reaction: This seems like a nice little success for analytical philosophy - clarifying a horrible mess by making a simple distinction that leaves everyone happy. |
| 5811 | A definite description can have a non-referential use [Donnellan] |
| Full Idea: A definite description may also be used non-referentially, even as it occurs in one and the same sentence. | |
| From: Keith Donnellan (Reference and Definite Descriptions [1966], §I) | |
| A reaction: Donnellan says we have to know about the particular occasion on which the description is used, as in itself it will not achieve reference. "Will the last person out switch off the lights" achieves its reference at the end of each day. |
| 5812 | Definite descriptions are 'attributive' if they say something about x, and 'referential' if they pick x out [Donnellan] |
| Full Idea: A speaker who uses a definite description 'attributively' in an assertion states something about whoever or whatever is the so-and-so; a speaker who uses it 'referentially' enables his audience to pick out whom or what he is talking about. | |
| From: Keith Donnellan (Reference and Definite Descriptions [1966], §III) | |
| A reaction: "Smith's murderer is insane" exemplifies the first use before he is caught, and the second use afterwards. The gist is that reference is not a purely linguistic activity, but is closer to pointing at something. This seems right. |
| 5814 | 'The x is F' only presumes that x exists; it does not actually entail the existence [Donnellan] |
| Full Idea: For Russell there is a logical entailment: 'the x is F' entails 'there exists one and only one x'. Whether or not this is true of the attributive use of definite descriptions, it does not seem true of the referential use. The existence is a presumption. | |
| From: Keith Donnellan (Reference and Definite Descriptions [1966], §VI) | |
| A reaction: Can we say 'x does not exist, but x is F'? Strictly, that sounds to me more like a contradiction than a surprising rejection of a presumption. However, 'Father Xmas does not exist, but he has a red coat'. |
| 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). |
| 10474 | |= should be read as 'is a model for' or 'satisfies' [Hodges,W] |
| Full Idea: The symbol in 'I |= S' reads that if the interpretation I (about word meaning) happens to make the sentence S state something true, then I 'is a model for' S, or I 'satisfies' S. | |
| From: Wilfrid Hodges (Model Theory [2005], 1) | |
| A reaction: Unfortunately this is not the only reading of the symbol |= [no space between | and =!], so care and familiarity are needed, but this is how to read it when dealing with models. See also Idea 10477. |
| 10473 | Model theory studies formal or natural language-interpretation using set-theory [Hodges,W] |
| Full Idea: Model theory is the study of the interpretation of any language, formal or natural, by means of set-theoretic structures, with Tarski's truth definition as a paradigm. | |
| From: Wilfrid Hodges (Model Theory [2005], Intro) | |
| A reaction: My attention is caught by the fact that natural languages are included. Might we say that science is model theory for English? That sounds like Quine's persistent message. |
| 10475 | A 'structure' is an interpretation specifying objects and classes of quantification [Hodges,W] |
| Full Idea: A 'structure' in model theory is an interpretation which explains what objects some expressions refer to, and what classes some quantifiers range over. | |
| From: Wilfrid Hodges (Model Theory [2005], 1) | |
| A reaction: He cites as examples 'first-order structures' used in mathematical model theory, and 'Kripke structures' used in model theory for modal logic. A structure is also called a 'universe'. |
| 10481 | Models in model theory are structures, not sets of descriptions [Hodges,W] |
| Full Idea: The models in model-theory are structures, but there is also a common use of 'model' to mean a formal theory which describes and explains a phenomenon, or plans to build it. | |
| From: Wilfrid Hodges (Model Theory [2005], 5) | |
| A reaction: Hodges is not at all clear here, but the idea seems to be that model-theory offers a set of objects and rules, where the common usage offers a set of descriptions. Model-theory needs homomorphisms to connect models to things, |
| 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. |
| 10480 | First-order logic can't discriminate between one infinite cardinal and another [Hodges,W] |
| Full Idea: First-order logic is hopeless for discriminating between one infinite cardinal and another. | |
| From: Wilfrid Hodges (Model Theory [2005], 4) | |
| A reaction: This seems rather significant, since mathematics largely relies on first-order logic for its metatheory. Personally I'm tempted to Ockham's Razor out all these super-infinities, but mathematicians seem to make use of them. |
| 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) |
| 3061 | Anaxarchus said that he was not even sure that he knew nothing [Anaxarchus, by Diog. Laertius] |
| Full Idea: Anaxarchus said that he was not even sure that he knew nothing. | |
| From: report of Anaxarchus (fragments/reports [c.340 BCE]) by Diogenes Laertius - Lives of Eminent Philosophers 09.10.1 |
| 10435 | A definite description 'the F' is referential if the speaker could thereby be referring to something not-F [Donnellan, by Sainsbury] |
| Full Idea: Donnellan argued that we could recognize a referential use of a definite description 'the F' by the fact that the speaker could thereby refer to something which is not F. | |
| From: report of Keith Donnellan (Reference and Definite Descriptions [1966]) by Mark Sainsbury - The Essence of Reference 18.5 | |
| A reaction: If the expression employed achieved reference whether the speaker wanted it to or not, it would certainly look as if the expression was inherently referring. |
| 10451 | Donnellan is unclear whether the referential-attributive distinction is semantic or pragmatic [Bach on Donnellan] |
| Full Idea: Donnellan seems to be unsure whether to regard his referential-attributive distinction as indicating a semantic ambiguity or merely a pragmatic one. | |
| From: comment on Keith Donnellan (Reference and Definite Descriptions [1966]) by Kent Bach - What Does It Take to Refer? 22.2 L1 | |
| A reaction: I vote for pragmatic. In a single brief conversation a definite description could start as attributive and end as referential, but it seems unlikely that its semantics changed in mid-paragraph. |
| 5813 | A description can successfully refer, even if its application to the subject is not believed [Donnellan] |
| Full Idea: If I think the king is a usurper, "Is the king in his counting house?" succeeds in referring to the right man, even though I do not believe that he fits the description. | |
| From: Keith Donnellan (Reference and Definite Descriptions [1966], §IV) | |
| A reaction: This seems undeniable. If I point at someone, I can refer successfully with almost any description. "Oy! Adolf! Get me a drink!" Reference is an essential aspect of language, and it is not entirely linguistic. |
| 5815 | Whether a definite description is referential or attributive depends on the speaker's intention [Donnellan] |
| Full Idea: Whether or not a definite description is used referentially or attributively is a function of the speaker's intentions in a particular case. | |
| From: Keith Donnellan (Reference and Definite Descriptions [1966], §VII) | |
| A reaction: Donnellan's distinction, and his claim here, seem to me right. However words on a notice could refer on one occasion, and just describe on another. "Anyone entering this cage is mad". |