17 ideas
| 13838 | A decent modern definition should always imply a semantics [Hacking] |
| Full Idea: Today we expect that anything worth calling a definition should imply a semantics. | |
| From: Ian Hacking (What is Logic? [1979], §10) | |
| A reaction: He compares this with Gentzen 1935, who was attempting purely syntactic definitions of the logical connectives. |
| 13833 | 'Thinning' ('dilution') is the key difference between deduction (which allows it) and induction [Hacking] |
| Full Idea: 'Dilution' (or 'Thinning') provides an essential contrast between deductive and inductive reasoning; for the introduction of new premises may spoil an inductive inference. | |
| From: Ian Hacking (What is Logic? [1979], §06.2) | |
| A reaction: That is, inductive logic (if there is such a thing) is clearly non-monotonic, whereas classical inductive logic is monotonic. |
| 13834 | Gentzen's Cut Rule (or transitivity of deduction) is 'If A |- B and B |- C, then A |- C' [Hacking] |
| Full Idea: If A |- B and B |- C, then A |- C. This generalises to: If Γ|-A,Θ and Γ,A |- Θ, then Γ |- Θ. Gentzen called this 'cut'. It is the transitivity of a deduction. | |
| From: Ian Hacking (What is Logic? [1979], §06.3) | |
| A reaction: I read the generalisation as 'If A can be either a premise or a conclusion, you can bypass it'. The first version is just transitivity (which by-passes the middle step). |
| 13835 | Only Cut reduces complexity, so logic is constructive without it, and it can be dispensed with [Hacking] |
| Full Idea: Only the cut rule can have a conclusion that is less complex than its premises. Hence when cut is not used, a derivation is quite literally constructive, building up from components. Any theorem obtained by cut can be obtained without it. | |
| From: Ian Hacking (What is Logic? [1979], §08) |
| 13845 | The various logics are abstractions made from terms like 'if...then' in English [Hacking] |
| Full Idea: I don't believe English is by nature classical or intuitionistic etc. These are abstractions made by logicians. Logicians attend to numerous different objects that might be served by 'If...then', like material conditional, strict or relevant implication. | |
| From: Ian Hacking (What is Logic? [1979], §15) | |
| A reaction: The idea that they are 'abstractions' is close to my heart. Abstractions from what? Surely 'if...then' has a standard character when employed in normal conversation? |
| 13840 | First-order logic is the strongest complete compact theory with Löwenheim-Skolem [Hacking] |
| Full Idea: First-order logic is the strongest complete compact theory with a Löwenheim-Skolem theorem. | |
| From: Ian Hacking (What is Logic? [1979], §13) |
| 13844 | A limitation of first-order logic is that it cannot handle branching quantifiers [Hacking] |
| Full Idea: Henkin proved that there is no first-order treatment of branching quantifiers, which do not seem to involve any idea that is fundamentally different from ordinary quantification. | |
| From: Ian Hacking (What is Logic? [1979], §13) | |
| A reaction: See Hacking for an example of branching quantifiers. Hacking is impressed by this as a real limitation of the first-order logic which he generally favours. |
| 13842 | Second-order completeness seems to need intensional entities and possible worlds [Hacking] |
| Full Idea: Second-order logic has no chance of a completeness theorem unless one ventures into intensional entities and possible worlds. | |
| From: Ian Hacking (What is Logic? [1979], §13) |
| 13837 | With a pure notion of truth and consequence, the meanings of connectives are fixed syntactically [Hacking] |
| Full Idea: My doctrine is that the peculiarity of the logical constants resides precisely in that given a certain pure notion of truth and consequence, all the desirable semantic properties of the constants are determined by their syntactic properties. | |
| From: Ian Hacking (What is Logic? [1979], §09) | |
| A reaction: He opposes this to Peacocke 1976, who claims that the logical connectives are essentially semantic in character, concerned with the preservation of truth. |
| 13839 | Perhaps variables could be dispensed with, by arrows joining places in the scope of quantifiers [Hacking] |
| Full Idea: For some purposes the variables of first-order logic can be regarded as prepositions and place-holders that could in principle be dispensed with, say by a system of arrows indicating what places fall in the scope of which quantifier. | |
| From: Ian Hacking (What is Logic? [1979], §11) | |
| A reaction: I tend to think of variables as either pronouns, or as definite descriptions, or as temporary names, but not as prepositions. Must address this new idea... |
| 13843 | If it is a logic, the Löwenheim-Skolem theorem holds for it [Hacking] |
| Full Idea: A Löwenheim-Skolem theorem holds for anything which, on my delineation, is a logic. | |
| From: Ian Hacking (What is Logic? [1979], §13) | |
| A reaction: I take this to be an unusually conservative view. Shapiro is the chap who can give you an alternative view of these things, or Boolos. |
| 468 | Musical performance can reveal a range of virtues [Damon of Ath.] |
| Full Idea: In singing and playing the lyre, a boy will be likely to reveal not only courage and moderation, but also justice. | |
| From: Damon (fragments/reports [c.460 BCE], B4), quoted by (who?) - where? |
| 22445 | Morality shows murder is wrong, but not what counts as a murder [Foot] |
| Full Idea: While one can determine from the concept of morality that there is an objection to murder one cannot determine completely what will count as murder. | |
| From: Philippa Foot (Morality and Art [1972], p.7) | |
| A reaction: She then refers to abortion, but there are military and criminal problem cases, and killings by neglect or side effect. |
| 22444 | A moral system must deal with the dangers and benefits of life [Foot] |
| Full Idea: A moral system seems necessarily to be one aimed at removing particular dangers and securing certain benefits. | |
| From: Philippa Foot (Morality and Art [1972], p.6) | |
| A reaction: I thoroughly approve of this approach to morality, which anchors it in real life, rather than in ideals or principles of reason. |
| 22447 | Saying something 'just is' right or wrong creates an illusion of fact and objectivity [Foot] |
| Full Idea: When we say that something 'just is' right or wrong we want to give the impression of some kind of fact or authority standing behind our words, ...maintaining the trappings of objectivity though the substance is not there. | |
| From: Philippa Foot (Morality and Art [1972], p.9) | |
| A reaction: Foot favours the idea that such a claim must depend on reasons, and that the reasons arise out of actual living. She's right. |
| 22448 | We sometimes just use the word 'should' to impose a rule of conduct on someone [Foot] |
| Full Idea: It would be more honest to recognise that the 'should' of moral judgement is sometimes merely an instrument by which we (for our own very good reasons) try to impose a rule of conduct even on the uncaring man? | |
| From: Philippa Foot (Morality and Art [1972], p.18) | |
| A reaction: This is a good example, I think, of the ordinary language tradition that Foot grew up in. We load a word like 'should' with a mystical power, but the situations in which it is actually used bring us back down to earth. |
| 22446 | In the case of something lacking independence, calling it a human being is a matter of choice [Foot] |
| Full Idea: In the problem of abortion there is a genuine choice as to whether or not to count as a human being, with the rights of a human being, what would become a human being but is not yet capable of independent life. | |
| From: Philippa Foot (Morality and Art [1972], p.7) | |
| A reaction: There must be some basis for the choice. We can't call a dead person a human being. Choosing to call a tiny zygote a human being seems very implausible. Pre-viability strikes me as implausible. |