26 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. |
| 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) |
| 2170 | Homer does not distinguish between soul and body [Homer, by Williams,B] |
| Full Idea: Homer's descriptions of people did without a dualistic distinction between soul and body. | |
| From: report of Homer (The Iliad [c.850 BCE]) by Bernard Williams - Shame and Necessity II - p.23 |
| 2171 | The 'will' doesn't exist; there is just conclusion, then action [Homer, by Williams,B] |
| Full Idea: Homer left out another mental action lying between coming to a conclusion and acting on it; and he did well, since there is no such action, and the idea is the invention of bad philosophy. | |
| From: report of Homer (The Iliad [c.850 BCE]) by Bernard Williams - Shame and Necessity II - p.37 | |
| A reaction: This is a characteristically empiricist view, which is found in Hobbes. The 'will' seems to have a useful role in folk psychology. We can at least say that coming to a conclusion that I should act, and then actually acting, are not the same thing. |
| 21819 | Plato says the Good produces the Intellectual-Principle, which in turn produces the Soul [Homer, by Plotinus] |
| Full Idea: In Plato the order of generation is from the Good, the Intellectual-Principle; from the Intellectual-Principle, the Soul. | |
| From: report of Homer (The Iliad [c.850 BCE], 509b) by Plotinus - The Enneads 5.1.08 | |
| A reaction: The doctrine of Plotinus merely echoes Plato, in that case, except that the One replaces the Form of the Good. Does this mean that what is first in Plotinus is less morally significant, and more concerned with reason and being? |
| 11388 | Let there be one ruler [Homer] |
| Full Idea: The rule of many is not good; let there be one ruler. | |
| From: Homer (The Iliad [c.850 BCE], 2.204), quoted by Vassilis Politis - Aristotle and the Metaphysics 8.9 | |
| A reaction: [Quoted by Aristotle at Metaphysics 1076a04] |
| 7257 | All modern social systems seem to be conspiracies of the rich [More,T] |
| Full Idea: When I consider any social system that prevails in the modern world, I can't see it as anything but a conspiracy of the rich to advance their own interests under the pretext of organizing society. | |
| From: Thomas More (Utopia [1516], Bk 2) | |
| A reaction: I'm afraid this is my own view of most conservative politics. I don't deny that there is a good case to be made for the conservative view (by Burke and Scruton, for example), but the rich will always latch onto its coat-tails. Cf. Idea 122. |
| 7254 | If you try to get elected, you should be permanently barred from seeking office [More,T] |
| Full Idea: In Utopia, anyone who deliberately tries to get himself elected to a public office is permanently disqualified from holding one. | |
| From: Thomas More (Utopia [1516], Bk 2) | |
| A reaction: This echoes a thought found in Plato (Idea 2149). I've always liked this idea. Why can't we have elections were a group of the best people are invited to stand? Well, yes, it would lead to corruption... Still, the best should be pushed to the front. |
| 7255 | Only Utopians fail to see glory in warfare [More,T] |
| Full Idea: Utopians are practically the only people on earth who fail to see anything glorious in war. | |
| From: Thomas More (Utopia [1516], Bk 2) | |
| A reaction: A refreshing thought for such an early date. Whatever dubious behaviour is nowadays attributed to Thomas More, you have to admire someone who writes this during the reign of Henry VIII. |
| 7253 | In Utopia, legal euthanasia is considered honourable [More,T] |
| Full Idea: In Utopia, officially sanctioned euthanasia is regarded as an honourable death. | |
| From: Thomas More (Utopia [1516], Bk 2) | |
| A reaction: A bit surprising coming from a writer who is now a Catholic martyr and saint. |
| 7256 | In Utopia, the Supreme Being is identical with Nature [More,T] |
| Full Idea: Everyone in Utopia agrees that the Supreme Being (which they call Mythras) is identical with Nature. | |
| From: Thomas More (Utopia [1516], Bk 2) | |
| A reaction: This sounds remarkably like full-blown Spinozean pantheism, though it should be interpreted with caution. It certainly seems to show that pantheism was a possibility in the minds of late medieval religious thinkers. |
| 14829 | Homer so enjoys the company of the gods that he must have been deeply irreligious [Homer, by Nietzsche] |
| Full Idea: Homer is so at home among his gods, and takes such delight in them as a poet, that he surely must have been deeply irreligious. | |
| From: report of Homer (The Iliad [c.850 BCE]) by Friedrich Nietzsche - Human, All Too Human 125 | |
| A reaction: Blake made a similar remark about where the true allegiance of Milton lay in 'Paradise Lost'. |