66 ideas
7910 | Pursue truth with the urgency of someone whose clothes are on fire [Ashvaghosha] |
Full Idea: As though your turban or your clothes were on fire, so with a sense of urgency should you apply your intellect to the comprehension of the truths. | |
From: Ashvaghosha (Saundaranandakavya [c.50], XVI) | |
A reaction: The best philosophers need no such urging. I retain a romantic view that we should be 'natural' in these things. See Plato's views in Idea 2153 and 1638. However, maybe I should be confronted with this quotation every morning when I awake. |
9724 | Until the 1960s the only semantics was truth-tables [Enderton] |
Full Idea: Until the 1960s standard truth-table semantics were the only ones that there were. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.10.1) | |
A reaction: The 1960s presumably marked the advent of possible worlds. |
15945 | Second-order set theory just adds a version of Replacement that quantifies over functions [Lavine] |
Full Idea: Second-order set theory is just like first-order set-theory, except that we use the version of Replacement with a universal second-order quantifier over functions from set to sets. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VII.4) |
9704 | 'ran R' indicates the 'range' of objects being related to [Enderton] |
Full Idea: 'ran R' indicates the 'range' of a relation, that is, the set of all objects that are members of ordered pairs and that are related to by the first objects. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9710 | We write F:A→B to indicate that A maps into B (the output of F on A is in B) [Enderton] |
Full Idea: We write F : A → B to indicate that A maps into B, that is, the domain of relating things is set A, and the things related to are all in B. If we add that F = B, then A maps 'onto' B. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9707 | 'F(x)' is the unique value which F assumes for a value of x [Enderton] |
Full Idea:
F(x) is a 'function', which indicates the unique value which y takes in |
|
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9703 | 'dom R' indicates the 'domain' of objects having a relation [Enderton] |
Full Idea: 'dom R' indicates the 'domain' of a relation, that is, the set of all objects that are members of ordered pairs and that have that relation. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9705 | 'fld R' indicates the 'field' of all objects in the relation [Enderton] |
Full Idea: 'fld R' indicates the 'field' of a relation, that is, the set of all objects that are members of ordered pairs on either side of the relation. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9712 | A relation is 'symmetric' on a set if every ordered pair has the relation in both directions [Enderton] |
Full Idea: A relation is 'symmetric' on a set if every ordered pair in the set has the relation in both directions. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9713 | A relation is 'transitive' if it can be carried over from two ordered pairs to a third [Enderton] |
Full Idea: A relation is 'transitive' on a set if the relation can be carried over from two ordered pairs to a third. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9699 | The 'powerset' of a set is all the subsets of a given set [Enderton] |
Full Idea: The 'powerset' of a set is all the subsets of a given set. Thus: PA = {x : x ⊆ A}. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9700 | Two sets are 'disjoint' iff their intersection is empty [Enderton] |
Full Idea: Two sets are 'disjoint' iff their intersection is empty (i.e. they have no members in common). | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9702 | A 'domain' of a relation is the set of members of ordered pairs in the relation [Enderton] |
Full Idea: The 'domain' of a relation is the set of all objects that are members of ordered pairs that are members of the relation. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9701 | A 'relation' is a set of ordered pairs [Enderton] |
Full Idea: A 'relation' is a set of ordered pairs. The ordering relation on the numbers 0-3 is captured by - in fact it is - the set of ordered pairs {<0,1>,<0,2>,<0,3>,<1,2>,<1,3>,<2,3>}. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) | |
A reaction: This can't quite be a definition of order among numbers, since it relies on the notion of a 'ordered' pair. |
9706 | A 'function' is a relation in which each object is related to just one other object [Enderton] |
Full Idea: A 'function' is a relation which is single-valued. That is, for each object, there is only one object in the function set to which that object is related. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9708 | A function 'maps A into B' if the relating things are set A, and the things related to are all in B [Enderton] |
Full Idea: A function 'maps A into B' if the domain of relating things is set A, and the things related to are all in B. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9709 | A function 'maps A onto B' if the relating things are set A, and the things related to are set B [Enderton] |
Full Idea: A function 'maps A onto B' if the domain of relating things is set A, and the things related to are set B. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9711 | A relation is 'reflexive' on a set if every member bears the relation to itself [Enderton] |
Full Idea: A relation is 'reflexive' on a set if every member of the set bears the relation to itself. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9714 | A relation satisfies 'trichotomy' if all pairs are either relations, or contain identical objects [Enderton] |
Full Idea: A relation satisfies 'trichotomy' on a set if every ordered pair is related (in either direction), or the objects are identical. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9717 | A set is 'dominated' by another if a one-to-one function maps the first set into a subset of the second [Enderton] |
Full Idea: A set is 'dominated' by another if a one-to-one function maps the first set into a subset of the second. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
15914 | An 'upper bound' is the greatest member of a subset; there may be several of these, so there is a 'least' one [Lavine] |
Full Idea: A member m of M is an 'upper bound' of a subset N of M if m is not less than any member of N. A member m of M is a 'least upper bound' of N if m is an upper bound of N such that if l is any other upper bound of N, then m is less than l. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.4) | |
A reaction: [if you don't follow that, you'll have to keep rereading it till you do] |
15921 | Collections of things can't be too big, but collections by a rule seem unlimited in size [Lavine] |
Full Idea: Since combinatorial collections are enumerated, some multiplicities may be too large to be gathered into combinatorial collections. But the size of a multiplicity seems quite irrelevant to whether it forms a logical connection. | |
From: Shaughan Lavine (Understanding the Infinite [1994], IV.2) |
15937 | Those who reject infinite collections also want to reject the Axiom of Choice [Lavine] |
Full Idea: Many of those who are skeptical about the existence of infinite combinatorial collections would want to doubt or deny the Axiom of Choice. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VI.2) |
9715 | An 'equivalence relation' is a reflexive, symmetric and transitive binary relation [Enderton] |
Full Idea: An 'equivalence relation' is a binary relation which is reflexive, and symmetric, and transitive. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
9716 | We 'partition' a set into distinct subsets, according to each relation on its objects [Enderton] |
Full Idea: Equivalence classes will 'partition' a set. That is, it will divide it into distinct subsets, according to each relation on the set. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], Ch.0) |
15936 | The Power Set is just the collection of functions from one collection to another [Lavine] |
Full Idea: The Power Set is just he codification of the fact that the collection of functions from a mathematical collection to a mathematical collection is itself a mathematical collection that can serve as a domain of mathematical study. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VI.1) |
15899 | Replacement was immediately accepted, despite having very few implications [Lavine] |
Full Idea: The Axiom of Replacement (of Skolem and Fraenkel) was remarkable for its universal acceptance, though it seemed to have no consequences except for the properties of the higher reaches of the Cantorian infinite. | |
From: Shaughan Lavine (Understanding the Infinite [1994], I) |
15930 | Foundation says descending chains are of finite length, blocking circularity, or ungrounded sets [Lavine] |
Full Idea: The Axiom of Foundation (Zermelo 1930) says 'Every (descending) chain in which each element is a member of the previous one is of finite length'. ..This forbids circles of membership, or ungrounded sets. ..The iterative conception gives this centre stage. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.4) |
15920 | Pure collections of things obey Choice, but collections defined by a rule may not [Lavine] |
Full Idea: Combinatorial collections (defined just by the members) obviously obey the Axiom of Choice, while it is at best dubious whether logical connections (defined by a rule) do. | |
From: Shaughan Lavine (Understanding the Infinite [1994], IV.2) |
15898 | The controversy was not about the Axiom of Choice, but about functions as arbitrary, or given by rules [Lavine] |
Full Idea: The controversy was not about Choice per se, but about the correct notion of function - between advocates of taking mathematics to be about arbitrary functions and advocates of taking it to be about functions given by rules. | |
From: Shaughan Lavine (Understanding the Infinite [1994], I) |
15919 | The 'logical' notion of class has some kind of definition or rule to characterise the class [Lavine] |
Full Idea: The Peano-Russell notion of class is the 'logical' notion, where each collection is associated with some kind of definition or rule that characterises the members of the collection. | |
From: Shaughan Lavine (Understanding the Infinite [1994], IV.1) |
15900 | The iterative conception of set wasn't suggested until 1947 [Lavine] |
Full Idea: The iterative conception of set was not so much as suggested, let alone advocated by anyone, until 1947. | |
From: Shaughan Lavine (Understanding the Infinite [1994], I) |
15931 | The iterative conception needs the Axiom of Infinity, to show how far we can iterate [Lavine] |
Full Idea: The iterative conception of sets does not tell us how far to iterate, and so we must start with an Axiom of Infinity. It also presupposes the notion of 'transfinite iteration'. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.5) |
15932 | The iterative conception doesn't unify the axioms, and has had little impact on mathematical proofs [Lavine] |
Full Idea: The iterative conception does not provide a conception that unifies the axioms of set theory, ...and it has had very little impact on what theorems can be proved. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.5) | |
A reaction: He says he would like to reject the iterative conception, but it may turn out that Foundation enables new proofs in mathematics (though it hasn't so far). |
15933 | Limitation of Size: if it's the same size as a set, it's a set; it uses Replacement [Lavine] |
Full Idea: Limitation of Size has it that if a collection is the same size as a set, then it is a set. The Axiom of Replacement is characteristic of limitation of size. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.5) |
15913 | A collection is 'well-ordered' if there is a least element, and all of its successors can be identified [Lavine] |
Full Idea: A collection M is 'well-ordered' by a relation < if < linearly orders M with a least element, and every subset of M that has an upper bound not in it has an immediate successor. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.4) |
9722 | Inference not from content, but from the fact that it was said, is 'conversational implicature' [Enderton] |
Full Idea: The process is dubbed 'conversational implicature' when the inference is not from the content of what has been said, but from the fact that it has been said. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.7.3) |
15926 | Second-order logic presupposes a set of relations already fixed by the first-order domain [Lavine] |
Full Idea: The distinctive feature of second-order logic is that it presupposes that, given a domain, there is a fact of the matter about what the relations on it are, so that the range of the second-order quantifiers is fixed as soon as the domain is fixed. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.3) | |
A reaction: This sounds like a rather large assumption, which is open to challenge. I am not sure whether it was the basis of Quine's challenge to second-order logic. He seems to have disliked its vagueness, because it didn't stick with 'objects'. |
9718 | Validity is either semantic (what preserves truth), or proof-theoretic (following procedures) [Enderton] |
Full Idea: The point of logic is to give an account of the notion of validity,..in two standard ways: the semantic way says that a valid inference preserves truth (symbol |=), and the proof-theoretic way is defined in terms of purely formal procedures (symbol |-). | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.1.3..) | |
A reaction: This division can be mirrored in mathematics, where it is either to do with counting or theorising about things in the physical world, or following sets of rules from axioms. Language can discuss reality, or play word-games. |
15934 | Mathematical proof by contradiction needs the law of excluded middle [Lavine] |
Full Idea: The Law of Excluded Middle is (part of) the foundation of the mathematical practice of employing proofs by contradiction. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VI.1) | |
A reaction: This applies in a lot of logic, as well as in mathematics. Come to think of it, it applies in Sudoku. |
9721 | A logical truth or tautology is a logical consequence of the empty set [Enderton] |
Full Idea: A is a logical truth (tautology) (|= A) iff it is a semantic consequence of the empty set of premises (φ |= A), that is, every interpretation makes A true. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.3.4) | |
A reaction: So the final column of every line of the truth table will be T. |
9994 | A truth assignment to the components of a wff 'satisfy' it if the wff is then True [Enderton] |
Full Idea: A truth assignment 'satisfies' a formula, or set of formulae, if it evaluates as True when all of its components have been assigned truth values. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.2) | |
A reaction: [very roughly what Enderton says!] The concept becomes most significant when a large set of wff's is pronounced 'satisfied' after a truth assignment leads to them all being true. |
9719 | A proof theory is 'sound' if its valid inferences entail semantic validity [Enderton] |
Full Idea: If every proof-theoretically valid inference is semantically valid (so that |- entails |=), the proof theory is said to be 'sound'. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.1.7) |
9720 | A proof theory is 'complete' if semantically valid inferences entail proof-theoretic validity [Enderton] |
Full Idea: If every semantically valid inference is proof-theoretically valid (so that |= entails |-), the proof-theory is said to be 'complete'. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.1.7) |
9995 | Proof in finite subsets is sufficient for proof in an infinite set [Enderton] |
Full Idea: If a wff is tautologically implied by a set of wff's, it is implied by a finite subset of them; and if every finite subset is satisfiable, then so is the whole set of wff's. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 2.5) | |
A reaction: [Enderton's account is more symbolic] He adds that this also applies to models. It is a 'theorem' because it can be proved. It is a major theorem in logic, because it brings the infinite under control, and who doesn't want that? |
9996 | Expressions are 'decidable' if inclusion in them (or not) can be proved [Enderton] |
Full Idea: A set of expressions is 'decidable' iff there exists an effective procedure (qv) that, given some expression, will decide whether or not the expression is included in the set (i.e. doesn't contradict it). | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.7) | |
A reaction: This is obviously a highly desirable feature for a really reliable system of expressions to possess. All finite sets are decidable, but some infinite sets are not. |
9997 | For a reasonable language, the set of valid wff's can always be enumerated [Enderton] |
Full Idea: The Enumerability Theorem says that for a reasonable language, the set of valid wff's can be effectively enumerated. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 2.5) | |
A reaction: There are criteria for what makes a 'reasonable' language (probably specified to ensure enumerability!). Predicates and functions must be decidable, and the language must be finite. |
15907 | Mathematics is nowadays (thanks to set theory) regarded as the study of structure, not of quantity [Lavine] |
Full Idea: Mathematics is today thought of as the study of abstract structure, not the study of quantity. That point of view arose directly out of the development of the set-theoretic notion of abstract structure. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.2) | |
A reaction: It sounds as if Structuralism, which is a controversial view in philosophy, is a fait accompli among mathematicians. |
15942 | Every rational number, unlike every natural number, is divisible by some other number [Lavine] |
Full Idea: One reason to introduce the rational numbers is that it simplifes the theory of division, since every rational number is divisible by every nonzero rational number, while the analogous statement is false for the natural numbers. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VI.3) | |
A reaction: That is, with rations every division operation has an answer. |
15922 | For the real numbers to form a set, we need the Continuum Hypothesis to be true [Lavine] |
Full Idea: The chief importance of the Continuum Hypothesis for Cantor (I believe) was that it would show that the real numbers form a set, and hence that they were encompassed by his theory. | |
From: Shaughan Lavine (Understanding the Infinite [1994], IV.2) |
18250 | Cauchy gave a necessary condition for the convergence of a sequence [Lavine] |
Full Idea: The Cauchy convergence criterion for a sequence: the sequence S0,S1,... has a limit if |S(n+r) - S(n)| is less than any given quantity for every value of r and sufficiently large values of n. He proved this necessary, but not sufficient. | |
From: Shaughan Lavine (Understanding the Infinite [1994], 2.5) |
15904 | The two sides of the Cut are, roughly, the bounding commensurable ratios [Lavine] |
Full Idea: Roughly speaking, the upper and lower parts of the Dedekind cut correspond to the commensurable ratios greater than and less than a given incommensurable ratio. | |
From: Shaughan Lavine (Understanding the Infinite [1994], II.6) | |
A reaction: Thus there is the problem of whether the contents of the gap are one unique thing, or many. |
15912 | Counting results in well-ordering, and well-ordering makes counting possible [Lavine] |
Full Idea: Counting a set produces a well-ordering of it. Conversely, if one has a well-ordering of a set, one can count it by following the well-ordering. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.4) | |
A reaction: Cantor didn't mean that you could literally count the set, only in principle. |
15949 | The theory of infinity must rest on our inability to distinguish between very large sizes [Lavine] |
Full Idea: The indiscernibility of indefinitely large sizes will be a critical part of the theory of indefinitely large sizes. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VIII.2) |
15947 | The infinite is extrapolation from the experience of indefinitely large size [Lavine] |
Full Idea: My proposal is that the concept of the infinite began with an extrapolation from the experience of indefinitely large size. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VIII.2) | |
A reaction: I think it might be better to talk of an 'abstraction' than an 'extrapolition', since the latter is just more of the same, which doesn't get you to concept. Lavine spends 100 pages working out his proposal. |
15940 | The intuitionist endorses only the potential infinite [Lavine] |
Full Idea: The intuitionist endorse the actual finite, but only the potential infinite. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VI.2) |
15909 | 'Aleph-0' is cardinality of the naturals, 'aleph-1' the next cardinal, 'aleph-ω' the ω-th cardinal [Lavine] |
Full Idea: The symbol 'aleph-nought' denotes the cardinal number of the set of natural numbers. The symbol 'aleph-one' denotes the next larger cardinal number. 'Aleph-omega' denotes the omega-th cardinal number. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.3) |
15915 | Ordinals are basic to Cantor's transfinite, to count the sets [Lavine] |
Full Idea: The ordinals are basic because the transfinite sets are those that can be counted, or (equivalently for Cantor), those that can be numbered by an ordinal or are well-ordered. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.4) | |
A reaction: Lavine observes (p.55) that for Cantor 'countable' meant 'countable by God'! |
15917 | Paradox: the class of all ordinals is well-ordered, so must have an ordinal as type - giving a bigger ordinal [Lavine] |
Full Idea: The paradox of the largest ordinal (the 'Burali-Forti') is that the class of all ordinal numbers is apparently well-ordered, and so it has an ordinal number as order type, which must be the largest ordinal - but all ordinals can be increased by one. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.5) |
15918 | Paradox: there is no largest cardinal, but the class of everything seems to be the largest [Lavine] |
Full Idea: The paradox of the largest cardinal ('Cantor's Paradox') says the diagonal argument shows there is no largest cardinal, but the class of all individuals (including the classes) must be the largest cardinal number. | |
From: Shaughan Lavine (Understanding the Infinite [1994], III.5) |
15929 | Set theory will found all of mathematics - except for the notion of proof [Lavine] |
Full Idea: Every theorem of mathematics has a counterpart with set theory - ...but that theory cannot serve as a basis for the notion of proof. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.3) |
15935 | Modern mathematics works up to isomorphism, and doesn't care what things 'really are' [Lavine] |
Full Idea: In modern mathematics virtually all work is only up to isomorphism and no one cares what the numbers or points and lines 'really are'. | |
From: Shaughan Lavine (Understanding the Infinite [1994], VI.1) | |
A reaction: At least that leaves the field open for philosophers, because we do care what things really are. So should everybody else, but there is no persuading some people. |
15928 | Intuitionism rejects set-theory to found mathematics [Lavine] |
Full Idea: Intuitionism in philosophy of mathematics rejects set-theoretic foundations. | |
From: Shaughan Lavine (Understanding the Infinite [1994], V.3 n33) |
9723 | Sentences with 'if' are only conditionals if they can read as A-implies-B [Enderton] |
Full Idea: Not all sentences using 'if' are conditionals. Consider 'if you want a banana, there is one in the kitchen'. The rough test is that a conditional can be rewritten as 'that A implies that B'. | |
From: Herbert B. Enderton (A Mathematical Introduction to Logic (2nd) [2001], 1.6.4) |
7909 | The Eightfold Path concerns morality, wisdom, and tranquillity [Ashvaghosha] |
Full Idea: The Eightfold Path has three steps concerning morality - right speech, right bodily action, and right livelihood; three of wisdom - right views, right intentions, and right effort; and two of tranquillity - right mindfulness and right concentration. | |
From: Ashvaghosha (Saundaranandakavya [c.50], XVI) | |
A reaction: Most of this translates quite comfortably into the aspirations of western philosophy. For example, 'right effort' sounds like Kant's claim that only a good will is truly good (Idea 3710). The Buddhist division is interesting for action theory. |
7908 | At the end of a saint, he is not located in space, but just ceases to be disturbed [Ashvaghosha] |
Full Idea: When an accomplished saint comes to the end, he does not go anywhere down in the earth or up in the sky, nor into any of the directions of space, but because his defilements have become extinct he simply ceases to be disturbed. | |
From: Ashvaghosha (Saundaranandakavya [c.50], XVI) | |
A reaction: To 'cease to be disturbed' is the most attractive account of heaven I have encountered. It all sounds a bit dull though. I wonder, as usual, how they know all this stuff. |