36 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) |
| 11211 | If a sound conclusion comes from two errors that cancel out, the path of the argument must matter [Rumfitt] |
| Full Idea: If a designated conclusion follows from the premisses, but the argument involves two howlers which cancel each other out, then the moral is that the path an argument takes from premisses to conclusion does matter to its logical evaluation. | |
| From: Ian Rumfitt ("Yes" and "No" [2000], II) | |
| A reaction: The drift of this is that our view of logic should be a little closer to the reasoning of ordinary language, and we should rely a little less on purely formal accounts. |
| 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) |
| 11210 | Standardly 'and' and 'but' are held to have the same sense by having the same truth table [Rumfitt] |
| Full Idea: If 'and' and 'but' really are alike in sense, in what might that likeness consist? Some philosophers of classical logic will reply that they share a sense by virtue of sharing a truth table. | |
| From: Ian Rumfitt ("Yes" and "No" [2000]) | |
| A reaction: This is the standard view which Rumfitt sets out to challenge. |
| 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. |
| 11212 | The sense of a connective comes from primitively obvious rules of inference [Rumfitt] |
| Full Idea: A connective will possess the sense that it has by virtue of its competent users' finding certain rules of inference involving it to be primitively obvious. | |
| From: Ian Rumfitt ("Yes" and "No" [2000], III) | |
| A reaction: Rumfitt cites Peacocke as endorsing this view, which characterises the logical connectives by their rules of usage rather than by their pure semantic value. |
| 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. |
| 23308 | Reasoning relates to understanding as time does to eternity [Boethius, by Sorabji] |
| Full Idea: Boethius says that reasoning [ratiocinatio] is related to intellectual understanding [intellectus] as time to eternity, involving as it does movement from one stage to another. | |
| From: report of Boethius (The Consolations of Philosophy [c.520], 4, prose 6) by Richard Sorabji - Rationality 'Shifting' | |
| A reaction: This gives true understanding a quasi-religious aura, as befits a subject which is truly consoling. |
| 5771 | Knowledge of present events doesn't make them necessary, so future events are no different [Boethius] |
| Full Idea: Just as the knowledge of present things imposes no necessity on what is happening, so foreknowledge imposes no necessity on what is going to happen. | |
| From: Boethius (The Consolations of Philosophy [c.520], V.IV) | |
| A reaction: This, I think, is the key idea if you are looking for a theological answer to the theological problem of free will. Don't think of God as seeing the future 'now'. God is outside time, and so only observes all of history just as we observe the present. |
| 5767 | Rational natures require free will, in order to have power of judgement [Boethius] |
| Full Idea: There is freedom of the will, for it would be impossible for any rational nature to exist without it. Whatever by nature has the use of reason has the power of judgement to decide each matter. | |
| From: Boethius (The Consolations of Philosophy [c.520], V.II) | |
| A reaction: A view taken up by Aquinas (Idea 1849) and Kant (Idea 3740). The 'power of judgement' pinpoints the core of rationality, and it is not clear how a robot could fulfil such a power, if it lacked consciousness. Does a machine 'judge' barcodes? |
| 5768 | God's universal foreknowledge seems opposed to free will [Boethius] |
| Full Idea: God's universal foreknowledge and freedom of the will seem clean contrary and opposite. | |
| From: Boethius (The Consolations of Philosophy [c.520], V.III) | |
| A reaction: The original source of the great theological and philosophical anguish over free will. The problem is anything which fixes future facts, be it oracular knowledge or scientific prediction. Personally I think free will was an invention by religions. |
| 5769 | Does foreknowledge cause necessity, or necessity cause foreknowledge? [Boethius] |
| Full Idea: Does foreknowledge of the future cause the necessity of events, or necessity cause the foreknowledge? | |
| From: Boethius (The Consolations of Philosophy [c.520], V.III) | |
| A reaction: An intriguing question, though not one that bothers me. I don't understand how foreknowledge causes necessity, unless God's vision of the future is a kind of 'freezing ray'. Even the gods must bow to necessity (Idea 3016). |
| 11214 | We learn 'not' along with affirmation, by learning to either affirm or deny a sentence [Rumfitt] |
| Full Idea: The standard view is that affirming not-A is more complex than affirming the atomic sentence A itself, with the latter determining its sense. But we could learn 'not' directly, by learning at once how to either affirm A or reject A. | |
| From: Ian Rumfitt ("Yes" and "No" [2000], IV) | |
| A reaction: [compressed] This seems fairly anti-Fregean in spirit, because it looks at the psychology of how we learn 'not' as a way of clarifying what we mean by it, rather than just looking at its logical behaviour (and thus giving it a secondary role). |
| 5762 | The wicked want goodness, so they would not be wicked if they obtained it [Boethius] |
| Full Idea: If the wicked obtained what they want - that is goodness - they could not be wicked. | |
| From: Boethius (The Consolations of Philosophy [c.520], IV.II) | |
| A reaction: This is a nice paradox which arises from Boethius being, like Socrates, an intellectualist. The question is whether the wicked want the good de re or de dicto. If they wanted to good de re (as its true self) they would obviously not be wicked. |
| 5770 | Rewards and punishments are not deserved if they don't arise from free movement of the mind [Boethius] |
| Full Idea: If there is no free will, then in vain is reward offered to the good and punishment to the bad, because they have not been deserved by any free and willed movement of the mind. | |
| From: Boethius (The Consolations of Philosophy [c.520], V.III) | |
| A reaction: I just don't see why decisions have to come out of nowhere in order to have any merit. People are different from natural forces, because the former can be persuaded by reasons. A moral agent is a mechanism which decides according to reasons. |
| 5764 | When people fall into wickedness they lose their human nature [Boethius] |
| Full Idea: When people fall into wickedness they lose their human nature. | |
| From: Boethius (The Consolations of Philosophy [c.520], IV.III) | |
| A reaction: This is a view I find quite sympathetic, but which is a million miles from the modern view. Today's paper showed a picture of a famous criminal holding a machine gun and a baby. We seem to delight in the idea that human nature is partly wicked. |
| 5756 | Happiness is a good which once obtained leaves nothing more to be desired [Boethius] |
| Full Idea: Happiness is a good which once obtained leaves nothing more to be desired. | |
| From: Boethius (The Consolations of Philosophy [c.520], III.I) | |
| A reaction: This sounds like the ancient 'eudaimonism' of Socrates and Aristotle, which might not be entirely compatible with orthodox Christianity. It is not true, though, that happy people lack ambition. To be happy, an unfilfilled aim may be needed. |
| 5763 | The bad seek the good through desire, but the good through virtue, which is more natural [Boethius] |
| Full Idea: The supreme good is the goal of good men and bad men alike, and the good seek it by means of a natural activity - the exercise of virtue - while the bad strive to acquire it by means of their desires, which is not a natural way of obtaining the good. | |
| From: Boethius (The Consolations of Philosophy [c.520], IV.II) | |
| A reaction: Interesting here is the slightly surprising claim that the pursuit of virtue is 'natural', implying that the mere pursuit of desire is not. Doesn't nature have to be restrained to achieve the good? Boethius is in the tradition of Aristotle and stoicism. |
| 5759 | Varied aims cannot be good because they differ, but only become good when they unify [Boethius] |
| Full Idea: The various things that men pursue are not perfect and good, because they differ from one another; ..when they differ they are not good, but when they begin to be one they become good, so it is through the acquisition of unity that these things are good. | |
| From: Boethius (The Consolations of Philosophy [c.520], III.XI) | |
| A reaction: This is a criticism of Aristotle's pluralism about the good(s) for man. Boethius' thought is appealing, and ties in with the Socratic notion that the virtues might be unified in some way. I think it is right that true virtues merge together, ideally. |
| 5754 | You can't control someone's free mind, only their body and possessions [Boethius] |
| Full Idea: The only way one man can exercise power over another is over his body and what is inferior to it, his possessions. You cannot impose anything on a free mind. | |
| From: Boethius (The Consolations of Philosophy [c.520], II.VI) | |
| A reaction: Written, of course, in prison. Boethius had not met hypnotism, or mind-controlling drugs, or invasive brain surgery. He hadn't read '1984'. He hadn't seen 'The Ipcress File'. (In fact, he should have got out more…) |
| 16692 | Divine eternity is the all-at-once and complete possession of unending life [Boethius] |
| Full Idea: Divine eternity is the all-at-once [tota simul] and complete possession of unending life. | |
| From: Boethius (The Consolations of Philosophy [c.520], V.6), quoted by Robert Pasnau - Metaphysical Themes 1274-1671 18.1 | |
| A reaction: This is a famous definition, and 'tota simul' became the phrase used for 'entia successiva', such as a day, or the Olympic Games. |
| 5752 | Where does evil come from if there is a god; where does good come from if there isn't? [Boethius] |
| Full Idea: A philosopher (possibly Epicurus) asked where evil comes from if there is a god, and where good comes from if there isn't. | |
| From: Boethius (The Consolations of Philosophy [c.520], I.IV) | |
| A reaction: A nice question. The best known answer to the first question is 'Satan'. Some would say that in the second case good is impossible, but I would have thought that the only possible answer is 'mankind'. |
| 5758 | God is the good [Boethius] |
| Full Idea: God is the good. | |
| From: Boethius (The Consolations of Philosophy [c.520], III.XI) | |
| A reaction: This summary follows on from the rather dubious discussion in Idea 5757. If God IS the good, it is not clear how God could be usefully described as 'good'. We would know that he was good a priori, without any enquiry into his nature being needed. |
| 5757 | God is the supreme good, so no source of goodness could take precedence over God [Boethius] |
| Full Idea: That which by its own nature is something distinct from supreme good, cannot be supreme good. ..It is impossible for anything to be by nature better than that from which it is derived, so that which is the origin of all things is supreme good. | |
| From: Boethius (The Consolations of Philosophy [c.520], III.X) | |
| A reaction: This is the contortion early Christians got into once they decided God had to be 'supreme' in the moral world (and every other world). Boethius allows a possible external source of all morality, but then has to say that this source is morally inferior. |
| 5760 | The power through which creation remains in existence and motion I call 'God' [Boethius] |
| Full Idea: For this power, whatever it is, through which creation remains in existence and in motion, I use the word which all people use, namely God. | |
| From: Boethius (The Consolations of Philosophy [c.520], III.XII) | |
| A reaction: An interesting caution in the phrase 'whatever it is'. Boethius would have been very open-minded in discussion with modern science about the stability of nature. Personally I reject Boethius' theory, but don't have a better one. Cf Idea 1431. |
| 5753 | The regular events of this life could never be due to chance [Boethius] |
| Full Idea: I could never believe that events of such regularity as we find in this life are due to the haphazards of chance. | |
| From: Boethius (The Consolations of Philosophy [c.520], I.VI) | |
| A reaction: It depends what you mean by 'chance'. Boethius infers a conscious mind, and presumes this to be God, but that is two large and unsupported steps. Modern atheists must acknowledge Boethius' problem. Why is there order? |
| 5765 | The reward of the good is to become gods [Boethius] |
| Full Idea: Goodness is happiness, ..but we agree that those who attain happiness are divine. The reward of the good, then, is to become gods. | |
| From: Boethius (The Consolations of Philosophy [c.520], IV.III) | |
| A reaction: Kant offered a similar argument (see Idea 1455). Most of us are unlikely to agree with the second premise of Boethius' argument. The idea that we might somehow become gods gripped the imagination for the next thousand years. |
| 5761 | God can do anything, but he cannot do evil, so evil must be nothing [Boethius] |
| Full Idea: 'There is nothing that an omnipotent power could not do?' 'No.' 'Then can God do evil?' 'No.' 'So evil is nothing, since that is what He cannot do who can do anthing.' | |
| From: Boethius (The Consolations of Philosophy [c.520], III.XII) | |
| A reaction: A lovely example of the contortions necessary once you insist that God must be 'omnipotent', in some absolute sense of the term. Saying that evil is 'nothing' strikes me as nothing more than a feeble attempt to insult it. |
| 5766 | If you could see the plan of Providence, you would not think there was evil anywhere [Boethius] |
| Full Idea: If you could see the plan of Providence, you would not think there was evil anywhere. | |
| From: Boethius (The Consolations of Philosophy [c.520], IV.VI) | |
| A reaction: This brings out the verificationist in me. See Idea 1467, by Antony Flew. Presumably Boethius would retain his faith as Europe moved horribly from 1939 to 1945, and even if the whole of humanity sank into squalid viciousness. |