18 ideas
| 17266 | Using modal logic, philosophers tried to handle all metaphysics in modal terms [Correia/Schnieder] |
| Full Idea: In the heyday of modal logic, philosophers typically tried to account for any metaphysical notions in modal terms. | |
| From: Correia,F/Schnieder,B (Grounding: an opinionated introduction [2012], 2.4) | |
| A reaction: Lewisian realism about possible worlds actually gets rid of purely 'modal' terms, but I suppose they include possible worlds in their remark. Annoying for modal logicians to be told they had a 'heyday' - a nice example of the rhetoric of philosophy. |
| 17263 | Why do rationalists accept Sufficient Reason, when it denies the existence of fundamental facts? [Correia/Schnieder] |
| Full Idea: What is most puzzling about the rationalist tradition is the steadfast certainty with which the Principle of Sufficient Reason was often accepted, since it in effect denies that there are fundamental facts. | |
| From: Correia,F/Schnieder,B (Grounding: an opinionated introduction [2012], 2.2) | |
| A reaction: A very simple and interesting observation. The principle implies either a circle of reasons, or an infinite regress of reasons. Nothing can be labelled as 'primitive' or 'foundational' or 'given'. The principle is irrational! |
| 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. |
| 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. |
| 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] |
| 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) |
| 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) |
| 10245 | One geometry cannot be more true than another [Poincaré] |
| Full Idea: One geometry cannot be more true than another; it can only be more convenient. | |
| From: Henri Poincaré (Science and Method [1908], p.65), quoted by Stewart Shapiro - Philosophy of Mathematics | |
| A reaction: This is the culminating view after new geometries were developed by tinkering with Euclid's parallels postulate. |
| 17270 | Is existential dependence by grounding, or do grounding claims arise from existential dependence? [Correia/Schnieder] |
| Full Idea: We may take existential dependence to be a relation induced by certain cases of grounding, but one may also think that facts about existential dependence are prior to corresponding ground claims, and in fact ground those claims. | |
| From: Correia,F/Schnieder,B (Grounding: an opinionated introduction [2012], 4.3) | |
| A reaction: I would vote for grounding, since dependence seems more abstract, and seems to demand explanation, whereas grounding seems more like a feature of reality, and to resist further intrinsic explanation (on the whole). |
| 17268 | Grounding is metaphysical and explanation epistemic, so keep them apart [Correia/Schnieder] |
| Full Idea: To us it seems advisable to separate the objective notion of grounding, which belongs to the field of metaphysics, from the epistemically loaded notion of explanation. | |
| From: Correia,F/Schnieder,B (Grounding: an opinionated introduction [2012], 4.2) | |
| A reaction: Paul Audi is the defender of the opposite view. I'm with Audi. The 'epistemically loaded' pragmatic aspect is just contextual - that we have different interests in different aspects of the grounding on different occasions. |
| 17267 | The identity of two facts may depend on how 'fine-grained' we think facts are [Correia/Schnieder] |
| Full Idea: There is a disagreement on the issue of factual identity, concerning the 'granularity' of facts, the question of how fine-grained they are. | |
| From: Correia,F/Schnieder,B (Grounding: an opinionated introduction [2012], 3.3) | |
| A reaction: If they are very fine-grained, then no two descriptions of a supposed fact will capture the same details. If we go broadbrush, facts become fuzzy and less helpful. 'Fact' was never going to be a clear term. |