23 ideas
| 10146 | Cantor's theories needed the Axiom of Choice, but it has led to great controversy [Feferman/Feferman] |
| Full Idea: The Axiom of Choice is a pure existence statement, without defining conditions. It was necessary to provide a foundation for Cantor's theory of transfinite cardinals and ordinal numbers, but its nonconstructive character engendered heated controversy. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int I) |
| 10147 | The Axiom of Choice is consistent with the other axioms of set theory [Feferman/Feferman] |
| Full Idea: In 1938 Gödel proved that the Axiom of Choice is consistent with the other axioms of set theory. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int I) | |
| A reaction: Hence people now standardly accept ZFC, rather than just ZF. |
| 10150 | The Trichotomy Principle is equivalent to the Axiom of Choice [Feferman/Feferman] |
| Full Idea: The Trichotomy Principle (any number is less, equal to, or greater than, another number) turned out to be equivalent to the Axiom of Choice. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int I) | |
| A reaction: [He credits Sierpinski (1918) with this discovery] |
| 10148 | Axiom of Choice: a set exists which chooses just one element each of any set of sets [Feferman/Feferman] |
| Full Idea: Zermelo's Axiom of Choice asserts that for any set of non-empty sets that (pairwise) have no elements in common, then there is a set that 'simultaneously chooses' exactly one element from each set. Note that this is an existential claim. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int I) | |
| A reaction: The Axiom is now widely accepted, after much debate in the early years. Even critics of the Axiom turn out to be relying on it. |
| 10149 | Platonist will accept the Axiom of Choice, but others want criteria of selection or definition [Feferman/Feferman] |
| Full Idea: The Axiom of Choice seems clearly true from the Platonistic point of view, independently of how sets may be defined, but is rejected by those who think such existential claims must show how to pick out or define the object claimed to exist. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int I) | |
| A reaction: The typical critics are likely to be intuitionists or formalists, who seek for both rigour and a plausible epistemology in our theory. |
| 23548 | Indeterminacy is in conflict with classical logic [Fine,K] |
| Full Idea: I now believe that the existence of indeterminacy is in conflict with classical logic. | |
| From: Kit Fine (Vagueness: a global approach [2020], 3) | |
| A reaction: I think that prior to this Fine had defended classical logic. Presumably the difficulty is over Bivalence. Nietzsche spotted this problem, despite not being a logician. Logic has to simplify the world. Hence philosophy is quite different from logic. |
| 23539 | Classical semantics has referents for names, extensions for predicates, and T or F for sentences [Fine,K] |
| Full Idea: A precise language is often assigned a classical semantics, in which the semantic value of a name is its referent, the semantic value of a predicate is its extension (the objects of which it is true), and the value of a sentence is True or False. | |
| From: Kit Fine (Vagueness: a global approach [2020], 1) | |
| A reaction: Helpful to have this clear statement of how predicates are treated. This extensionalism in logic causes trouble when it creeps into philosophy, and people say that 'red' just means all the red things. No it doesn't. |
| 10158 | A structure is a 'model' when the axioms are true. So which of the structures are models? [Feferman/Feferman] |
| Full Idea: A structure is said to be a 'model' of an axiom system if each of its axioms is true in the structure (e.g. Euclidean or non-Euclidean geometry). 'Model theory' concerns which structures are models of a given language and axiom system. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V) | |
| A reaction: This strikes me as the most interesting aspect of mathematical logic, since it concerns the ways in which syntactic proof-systems actually connect with reality. Tarski is the central theoretician here, and his theory of truth is the key. |
| 10162 | Tarski and Vaught established the equivalence relations between first-order structures [Feferman/Feferman] |
| Full Idea: In the late 1950s Tarski and Vaught defined and established basic properties of the relation of elementary equivalence between two structures, which holds when they make true exactly the same first-order sentences. This is fundamental to model theory. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V) | |
| A reaction: This is isomorphism, which clarifies what a model is by giving identity conditions between two models. Note that it is 'first-order', and presumably founded on classical logic. |
| 10160 | Löwenheim-Skolem says if the sentences are countable, so is the model [Feferman/Feferman] |
| Full Idea: The Löwenheim-Skolem Theorem, the earliest in model theory, states that if a countable set of sentences in a first-order language has a model, then it has a countable model. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V) | |
| A reaction: There are 'upward' (sentences-to-model) and 'downward' (model-to-sentences) versions of the theory. |
| 10159 | Löwenheim-Skolem Theorem, and Gödel's completeness of first-order logic, the earliest model theory [Feferman/Feferman] |
| Full Idea: Before Tarski's work in the 1930s, the main results in model theory were the Löwenheim-Skolem Theorem, and Gödel's establishment in 1929 of the completeness of the axioms and rules for the classical first-order predicate (or quantificational) calculus. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V) |
| 10161 | If a sentence holds in every model of a theory, then it is logically derivable from the theory [Feferman/Feferman] |
| Full Idea: Completeness is when, if a sentences holds in every model of a theory, then it is logically derivable from that theory. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int V) |
| 10156 | 'Recursion theory' concerns what can be solved by computing machines [Feferman/Feferman] |
| Full Idea: 'Recursion theory' is the subject of what can and cannot be solved by computing machines | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Ch.9) | |
| A reaction: This because 'recursion' will grind out a result step-by-step, as long as the steps will 'halt' eventually. |
| 10155 | Both Principia Mathematica and Peano Arithmetic are undecidable [Feferman/Feferman] |
| Full Idea: In 1936 Church showed that Principia Mathematica is undecidable if it is ω-consistent, and a year later Rosser showed that Peano Arithmetic is undecidable, and any consistent extension of it. | |
| From: Feferman / Feferman (Alfred Tarski: life and logic [2004], Int IV) |
| 23544 | Local indeterminacy concerns a single object, and global indeterminacy covers a range [Fine,K] |
| Full Idea: Vagueness concerns 'local' indeterminacy, such as whether one man in the lineup is bald, and 'global' indeterminacy, applying to a range of cases, as when it is indeterminate how 'bald' applies to the lineup. But how do these relate? | |
| From: Kit Fine (Vagueness: a global approach [2020], 1) | |
| A reaction: This puts the focus either on objects or on predicates which are vague. |
| 23540 | Conjoining two indefinites by related sentences seems to produce a contradiction [Fine,K] |
| Full Idea: If 'P is red' and 'P is orange' are indefinite, then 'P is red and P is orange' seems false, because red and orange are exclusive. But if two conjoined indefinite sentences are false, that makes 'P is red and P is red' false, when it should be indefinite. | |
| From: Kit Fine (Vagueness: a global approach [2020], 1) | |
| A reaction: [compressed] This is the problem of 'penumbral connection', where two indefinite values are still logically related, by excluding one another. Presumably 'P is red and P is of indefinite shape' can be true? Doubtful about this argument. |
| 23546 | Standardly vagueness involves borderline cases, and a higher standpoint from which they can be seen [Fine,K] |
| Full Idea: Standard notions of vagueness all accept borderline cases, and presuppose a higher standpoint from which a judgement of being borderline F, rather than simply being F or being not F, can be made. | |
| From: Kit Fine (Vagueness: a global approach [2020], 3) | |
| A reaction: He says that the concept of borderline cases is an impediment to understanding vagueness. Proposing a third group when you are struggling to separate two other groups doesn't seem helpful, come to think of it. Limbo cases. |
| 23542 | Identifying vagueness with ignorance is the common mistake of confusing symptoms with cause [Fine,K] |
| Full Idea: We can see Epistemicism [vagueness as ignorance] as a common and misguided tendency to identify a cause with its symptoms. We are unsure how to characterise vagueness, and identify it with the resulting ignorance, instead of explaining it. | |
| From: Kit Fine (Vagueness: a global approach [2020], 1) | |
| A reaction: Love it. This echoes my repeated plea in these reactions to stop identifying features of reality with the functions which embody them or the patterns they create. We need to explain them, and must dig deeper. |
| 23541 | Supervaluation can give no answer to 'who is the last bald man' [Fine,K] |
| Full Idea: Under supervaluation there should always be someone who is the last bald man in the sequence, but there is always an acceptable way to make some other man the last bald man. | |
| From: Kit Fine (Vagueness: a global approach [2020], 1) | |
| A reaction: Fine seems to take this as a conclusive refutation of the supervaluation approach. Fine says (p.41) that supervaluation says there is a precisification for every instance. |
| 23545 | We do not have an intelligible concept of a borderline case [Fine,K] |
| Full Idea: We simply have no intelligible notion of local indeterminacy or of a borderline case. | |
| From: Kit Fine (Vagueness: a global approach [2020], 2) | |
| A reaction: He mentions cases which are near a borderline, and cases which are hard to decide, but denies that these are intrinsically borderline. If there are borderline cases between red and orange, what are the outer boundaries of the border? |
| 18284 | Particulars can be verified or falsified, but general statements can only be falsified (conclusively) [Popper] |
| Full Idea: Whereas particular reality statements are in principle completely verifiable or falsifiable, things are different for general reality statements: they can indeed be conclusively falsified, they can acquire a negative truth value, but not a positive one. | |
| From: Karl Popper (Two Problems of Epistemology [1932], p.256), quoted by J. Alberto Coffa - The Semantic Tradition from Kant to Carnap 18 'Laws' | |
| A reaction: This sounds like a logician's approach to science, but I prefer to look at coherence, where very little is actually conclusive, and one tinkers with the theory instead. |
| 23547 | It seems absurd that there is no identity of any kind between two objects which involve survival [Fine,K] |
| Full Idea: Pace Parfit and others, it boggles the mind that survival could be independent of any relation of identity between the currently existing object and the objects that subsequently exist. | |
| From: Kit Fine (Vagueness: a global approach [2020], 3) | |
| A reaction: Yes. If the self or mind just consists of a diachronic trail of memories such that the two ends of the trail have no connection at all, that isn't the kind of survival that any of us want. I want to live my life, not a life. |
| 23543 | We identify laws with regularities because we mistakenly identify causes with their symptoms [Fine,K] |
| Full Idea: There is a common tendency to identify a cause with its symptoms. Hence we are not sure how to characterise a law, and so we identify it with the regularities to which it gives rise. | |
| From: Kit Fine (Vagueness: a global approach [2020], 1) | |
| A reaction: A lovely clear identification of my pet hate, which is superficial accounts of things, which claim to be the last word, but actually explain nothing. |