18 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. |
| 2614 | Modern phenomenalism holds that objects are logical constructions out of sense-data [Ayer] |
| Full Idea: Nowadays phenomenalism is held to be a theory of perception which says that physical objects are logical constructions out of sense-data. | |
| From: A.J. Ayer (Phenomenalism [1947], §1) |
| 2615 | The concept of sense-data allows us to discuss appearances without worrying about reality [Ayer] |
| Full Idea: The introduction of the term 'sense-datum' is a means of referring to appearances without prejudging the question of what it is, if anything, that they are appearances of. | |
| From: A.J. Ayer (Phenomenalism [1947], §1) |
| 9261 | The 'Ethics' is disappointing, because it fails to try to justify our duties [Prichard] |
| Full Idea: Reading the 'Ethics' is so disappointing, because Aristotle does not try to convince us that we really ought to do what our non-reflective consciousness has hitherto believed we ought to do. | |
| From: H.A. Prichard (Does moral phil rest on a mistake? [1912]) | |
| A reaction: Aristotle didn't speak the language of 'duty' (see Idea 2172), but he could work it into his account if Prichard asked nicely. I take the truly virtuous person to be, above all, a wonderful citizen. Duties are contractual; good deeds flow from virtue. |
| 9262 | The mistake is to think we can prove what can only be seen directly in moral thinking [Prichard] |
| Full Idea: Moral Philosophy rests on the mistake of supposing the possibility of proving what can only be apprehended directly by an act of moral thinking. | |
| From: H.A. Prichard (Does moral phil rest on a mistake? [1912]) | |
| A reaction: This is a beginning of the rebellion against the Enlightenment Project in ethics, which is why Prichard has become popular. At bottom he is offering intuition ('direct moral thinking'), which is a frustratingly thin concept. |
| 9260 | Virtues won't generate an obligation, so it isn't a basis for morality [Prichard] |
| Full Idea: It is untrue to urge that, since courage is a virtue, we ought to act courageously. We feel an obligation to act, but not from a certain desire. The action is done from obligation, so isn't an act of courage. ..In fact, virtue is no basis for morality. | |
| From: H.A. Prichard (Does moral phil rest on a mistake? [1912]) | |
| A reaction: One of the few interesting and direct attacks on virtue theory, before its modern revival. Prichard urges a perception of what is valuable (or good) as the basis for obligation and right action. He is right that values come first, in virtue and elsewhere. |
| 9259 | We feel obligations to overcome our own failings, and these are not relations to other people [Prichard] |
| Full Idea: The relation involved in an obligation need not be a relation to another at all. Thus we should admit that there is an obligation to overcome our natural timidity or greediness, and this involves no relations to others. | |
| From: H.A. Prichard (Does moral phil rest on a mistake? [1912]) | |
| A reaction: An interesting un-Aristotelian and individualistic view of virtue. Why would we want to rid ourselves of timidity or greediness? Either it is self-interested, or we wish to be better citizens. See Richard Taylor on duty. |
| 9258 | If pain were instrinsically wrong, it would be immoral to inflict it on ourselves [Prichard] |
| Full Idea: If the badness of pain were the reason why we ought not to inflict pain on another, it would equally be a reason why we ought not to inflict pain on ourselves; yet, though we would call such behaviour foolish, we wouldn't think it wrong. | |
| From: H.A. Prichard (Does moral phil rest on a mistake? [1912], n4) | |
| A reaction: A very nice point. Note that it will equally well apply to 'benefit' or 'preferences', or any other ideal which utilitarians set out to maximise. It may not be bad to hurt yourself, but it might still be bad to harm yourself. |