15 ideas
| 10163 | Propositional modal logic has been proved to be complete [Kripke, by Feferman/Feferman] |
| Full Idea: At the age of 19 Saul Kripke published a completeness proof of propositional modal logic. | |
| From: report of Saul A. Kripke (A Completeness Theorem in Modal Logic [1959]) by Feferman / Feferman - Alfred Tarski: life and logic Int V |
| 10760 | With possible worlds, S4 and S5 are sound and complete, but S1-S3 are not even sound [Kripke, by Rossberg] |
| Full Idea: Kripke gave a possible worlds semantics to a whole range of modal logics, and S4 and S5 turned out to be both sound and complete with this semantics. Hence more systems could be designed. S1-S3 failed in soundness, leading to 'impossible worlds'. | |
| From: report of Saul A. Kripke (A Completeness Theorem in Modal Logic [1959]) by Marcus Rossberg - First-order Logic, 2nd-order, Completeness §4 |
| 16189 | The variable domain approach to quantified modal logic invalidates the Barcan Formula [Kripke, by Simchen] |
| Full Idea: Kripke's variable domain approach to quantified modal logic famously invalidates the Barcan Formula. | |
| From: report of Saul A. Kripke (A Completeness Theorem in Modal Logic [1959]) by Ori Simchen - The Barcan Formula and Metaphysics §3 | |
| A reaction: [p.9 and p.16] In a single combined domain all the possibilia must be present, but with variable domains objects in remote domains may not exist in your local domain. BF is committed to those possible objects. |
| 15132 | The Barcan formulas fail in models with varying domains [Kripke, by Williamson] |
| Full Idea: Kripke showed that the Barcan formula ∀x□A⊃□∀xA and its converse fail in models which require varying domains. | |
| From: report of Saul A. Kripke (A Completeness Theorem in Modal Logic [1959]) by Timothy Williamson - Truthmakers and Converse Barcan Formula §1 | |
| A reaction: I think this is why I reject the Barcan formulas for metaphysics - because the domain of metaphysics should be seen as varying, since some objects are possible in some contexts and not in others. Hmm… |
| 17884 | Mathematical set theory has many plausible stopping points, such as finitism, and predicativism [Koellner] |
| Full Idea: There are many coherent stopping points in the hierarchy of increasingly strong mathematical systems, starting with strict finitism, and moving up through predicativism to the higher reaches of set theory. | |
| From: Peter Koellner (On the Question of Absolute Undecidability [2006], Intro) |
| 17893 | 'Reflection principles' say the whole truth about sets can't be captured [Koellner] |
| Full Idea: Roughly speaking, 'reflection principles' assert that anything true in V [the set hierarchy] falls short of characterising V in that it is true within some earlier level. | |
| From: Peter Koellner (On the Question of Absolute Undecidability [2006], 2.1) |
| 17894 | We have no argument to show a statement is absolutely undecidable [Koellner] |
| Full Idea: There is at present no solid argument to the effect that a given statement is absolutely undecidable. | |
| From: Peter Koellner (On the Question of Absolute Undecidability [2006], 5.3) |
| 17890 | There are at least eleven types of large cardinal, of increasing logical strength [Koellner] |
| Full Idea: Some of the standard large cardinals (in order of increasing (logical) strength) are: inaccessible, Mahlo, weakly compact, indescribable, Erdös, measurable, strong, Wodin, supercompact, huge etc. (...and ineffable). | |
| From: Peter Koellner (On the Question of Absolute Undecidability [2006], 1.4) | |
| A reaction: [I don't understand how cardinals can have 'logical strength', but I pass it on anyway] |
| 17887 | PA is consistent as far as we can accept, and we expand axioms to overcome limitations [Koellner] |
| Full Idea: To the extent that we are justified in accepting Peano Arithmetic we are justified in accepting its consistency, and so we know how to expand the axiom system so as to overcome the limitation [of Gödel's Second Theorem]. | |
| From: Peter Koellner (On the Question of Absolute Undecidability [2006], 1.1) | |
| A reaction: Each expansion brings a limitation, but then you can expand again. |
| 17891 | Arithmetical undecidability is always settled at the next stage up [Koellner] |
| Full Idea: The arithmetical instances of undecidability that arise at one stage of the hierarchy are settled at the next. | |
| From: Peter Koellner (On the Question of Absolute Undecidability [2006], 1.4) |
| 20180 | A happy and joyous life must largely be a quiet life [Russell] |
| Full Idea: A happy life must to a great extent be a quiet life, for it is only in an atmosphere of quiet that true joy can live. | |
| From: Bertrand Russell (The Conquest of Happiness [1930], 4) | |
| A reaction: Most people's image of happiness is absorption in an interesting task, or relaxing in good company. The idea that happiness is wild excitement exists, but is a minority view. |
| 20177 | Boredom always involves not being fully occupied [Russell] |
| Full Idea: It is one of the essentials of boredom that one's faculties must not be fully occupied. | |
| From: Bertrand Russell (The Conquest of Happiness [1930], 4) | |
| A reaction: He gives running for your life as an example of non-boredom. I suspect that this is only the sort of boredom that troubled Russell, and not the sort of profound boredom that led the actor George Sanders to suicide (according to his last note). |
| 20179 | Happiness involves enduring boredom, and the young should be taught this [Russell] |
| Full Idea: A certain power of enduring boredom is essential to a happy life, and is one of the things that ought to be taught to the young. | |
| From: Bertrand Russell (The Conquest of Happiness [1930], 4) | |
| A reaction: As an example he suggests that Wordsworth would never have written 'The Prelude' is he had never been bored when young. Which suggests that Russell doesn't really get boredom, seeing it merely as a stimulus to work. |
| 20176 | Boredom is an increasingly strong motivating power [Russell] |
| Full Idea: Boredom has been, I believe, one of the great motive powers throughout the historical epoch, and is so at the present day more than ever. | |
| From: Bertrand Russell (The Conquest of Happiness [1930], 4) | |
| A reaction: Most of his essay tells us how to avoid boredom, rather than how it motivates. |
| 20178 | Life is now more interesting, but boredom is more frightening [Russell] |
| Full Idea: We are less bored than our ancestors were, but we are more afraid of boredom | |
| From: Bertrand Russell (The Conquest of Happiness [1930], 4) | |
| A reaction: I get the impression that the invention of the powerful mobile phone has largely banished boredom from human life, except when you are obliged to switch it off. The fear of boredom may hence be even greater now. |