19 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. |
| 7544 | Many people imagine that to experience is to understand [Goethe] |
| Full Idea: There are many people who imagine that what they experience they also understand. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 889) | |
| A reaction: This should be posted over the arrivals gate of every international airport, for returning holiday-makers. It seems to place Goethe on the rationalist side of the debate with empiricism. It is hard to explain 'understanding' in Humean terms. |
| 1556 | By nature people are close to one another, but culture drives them apart [Hippias] |
| Full Idea: I regard you all as relatives - by nature, not by convention. By nature like is akin to like, but convention is a tyrant over humankind and often constrains people to act contrary to nature. | |
| From: Hippias (fragments/reports [c.430 BCE]), quoted by Plato - Protagoras 337c8 |
| 7541 | Man never understands how anthropomorphic he is [Goethe] |
| Full Idea: Man never understands how anthropomorphic he is. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 203) | |
| A reaction: Nice. It is true, even when it is pointed out to us. No matter how hard we try to realise how very different animals are from us, we can't help identifying with them. Religious people even do it with inanimate creation. |
| 7543 | We gain self-knowledge through action, not thought - especially when doing our duty [Goethe] |
| Full Idea: How can we learn self-knowledge? Never by taking thought, but rather by action. Try to do your duty and you'll soon discover what you're like. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 442) | |
| A reaction: Good! I even like the unfashionable bit about duty. If you just do what you want, you will discover your interests, but not so much about your capacities. However, when you have to do something less comfortable, it is very revealing. |
| 7540 | Beauty is a manifestation of secret natural laws [Goethe] |
| Full Idea: Beauty is a manifestation of secret natural laws which without this appearance would have remained eternally hidden from us. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 183) | |
| A reaction: An interesting defence of beauty as an objective feature of the world. I'm not sure. Much beauty is indeed the result of growth or erosion expressing underlying laws, but then I have always thought there was a sexual component to visual beauty. |
| 7538 | The happiest people link the beginning and end of life [Goethe] |
| Full Idea: The happiest man is one who can link the end of his life with its beginning. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 140) | |
| A reaction: [from 'Art and Antiquity']. A nice thought, which chimes in with the idea that a good life is like a complete story or a work of art (Idea 7501), or that it is 'eudaimon'. |
| 7542 | The best form of government teaches us to govern ourselves [Goethe] |
| Full Idea: You ask which form of government is the best? Whichever teaches us to govern ourselves. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 353) | |
| A reaction: Not a fashionable view, since the rise of freedom as the highest political ideal, but I identify with the idea that a good government should educate, and should try to facilitate virtue as well as pleasure. |
| 7539 | To get duties from people without rights, you must pay them well [Goethe] |
| Full Idea: If you demand duties from people and will not concede them rights, you have to pay them well. | |
| From: Wolfgang von Goethe (Maxims and Reflections [1825], 180) | |
| A reaction: [from 'Art and Antiquity']. ...or have great power over them. Goethe gives the optimistic liberal view, rather than the Marxist view. |