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6. Mathematics / B. Foundations for Mathematics / 5. Definitions of Number / c. Fregean numbers

[Frege's view of numbers as extensions of classes]

46 ideas
The 'extension of a concept' in general may be quantitatively completely indeterminate [Cantor]
     Full Idea: The author entirely overlooks the fact that the 'extension of a concept' in general may be quantitatively completely indeterminate. Only in certain cases is the 'extension of a concept' quantitatively determinate.
     From: George Cantor (Review of Frege's 'Grundlagen' [1885], 1932:440), quoted by William W. Tait - Frege versus Cantor and Dedekind
     A reaction: Cantor presumably has in mind various infinite sets. Tait is drawing our attention to the fact that this objection long precedes Russell's paradox, which made the objection more formal (a language Frege could understand!).
There is the concept, the object falling under it, and the extension (a set, which is also an object) [Frege, by George/Velleman]
     Full Idea: For Frege, the extension of the concept F is an object, as revealed by the fact that we use a name to refer to it. ..We must distinguish the concept, the object that falls under it, and the extension of the concept, which is the set containing the object.
     From: report of Gottlob Frege (On Concept and Object [1892]) by A.George / D.J.Velleman - Philosophies of Mathematics Ch.2
     A reaction: This I take to be the key distinction needed if one is to grasp Frege's account of what a number is. When we say that Frege is a platonist about numbers, it is because he is committed to the notion that the extension is an object.
Frege defined number in terms of extensions of concepts, but needed Basic Law V to explain extensions [Frege, by Hale/Wright]
     Full Idea: Frege opts for his famous definition of numbers in terms of extensions of the concept 'equal to the concept F', but he then (in 'Grundgesetze') needs a theory of extensions or classes, which he provided by means of Basic Law V.
     From: report of Gottlob Frege (Grundgesetze der Arithmetik 1 (Basic Laws) [1893]) by B Hale / C Wright - Intro to 'The Reason's Proper Study' §1
Frege ignored Cantor's warning that a cardinal set is not just a concept-extension [Tait on Frege]
     Full Idea: Cantor pointed out explicitly to Frege that it is a mistake to take the notion of a set (i.e. of that which has a cardinal number) to simply mean the extension of a concept. ...Frege's later assumption of this was an act of recklessness.
     From: comment on Gottlob Frege (Grundgesetze der Arithmetik 1 (Basic Laws) [1893]) by William W. Tait - Frege versus Cantor and Dedekind III
     A reaction: ['recklessness' is on p.61] Tait has no sympathy with the image of Frege as an intellectual martyr. Frege had insufficient respect for a great genius. Cantor, crucially, understood infinity much better than Frege.
Frege's biggest error is in not accounting for the senses of number terms [Hodes on Frege]
     Full Idea: The inconsistency of Grundgesetze was only a minor flaw. Its fundamental flaw was its inability to account for the way in which the senses of number terms are determined. It leaves the reference-magnetic nature of the standard numberer a mystery.
     From: comment on Gottlob Frege (Grundgesetze der Arithmetik 2 (Basic Laws) [1903]) by Harold Hodes - Logicism and Ontological Commits. of Arithmetic p.139
     A reaction: A point also made by Hofweber. As a logician, Frege was only concerned with the inferential role of number terms, and he felt he had captured their logical form, but it is when you come to look at numbers in natural language that he seem in trouble.
A number is a class of classes of the same cardinality [Frege, by Dummett]
     Full Idea: For Frege, in 'Grundgesetze', a number is a class of classes of the same cardinality.
     From: report of Gottlob Frege (Grundgesetze der Arithmetik 2 (Basic Laws) [1903]) by Michael Dummett - Frege Philosophy of Language (2nd ed) Ch.14
Frege had a motive to treat numbers as objects, but not a justification [Hale/Wright on Frege]
     Full Idea: It has been observed that Frege has a motive to treat numbers as objects, but not a justification for doing so.
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by B Hale / C Wright - Intro to 'The Reason's Proper Study' §3.2
If you can subdivide objects many ways for counting, you can do that to set-elements too [Yourgrau on Frege]
     Full Idea: If we are allowed in the case of sets to construe the number question as 'really': How many (elements)?, then we could just as well construe Frege's famous question about the deck of cards as: How many (cards)?
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Palle Yourgrau - Sets, Aggregates and Numbers 'New Problem'
     A reaction: My view is that counting is not entirely relative to the concept employed, but that the world imposes objects on us which thus impose their concepts and their counting. This is 'natural', but we can then counter nature with pragmatics and whimsy.
Frege claims that numbers are objects, as opposed to them being Fregean concepts [Frege, by Wright,C]
     Full Idea: When Frege urges that numbers are to be thought of as objects, the content of this claim has to be derived from its opposition to the claim that numbers are Fregean concepts.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Crispin Wright - Frege's Concept of Numbers as Objects 1.ii
Numbers are second-level, ascribing properties to concepts rather than to objects [Frege, by Wright,C]
     Full Idea: Frege had the insight that statements of number, like statements of existence, are in a sense second-level. That is, they are most fruitfully and least confusingly seen as ascribing a property not to an object, but to a concept.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Crispin Wright - Frege's Concept of Numbers as Objects 1.iii
     A reaction: This sounds neat, but I'm immediately wondering whether he is just noticing how languages work, rather than how things are. If I say red is a bright colour, I am saying something about red objects.
For Frege, successor was a relation, not a function [Frege, by Dummett]
     Full Idea: Frege was operating with a successor relation, rather than a successor function.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.2
     A reaction: That is, succession is a given fact, not a construction. 4 may be the successor of 3 in natural numbers, but not in rational or real numbers, so we can't take the relation for granted.
A cardinal number may be defined as a class of similar classes [Frege, by Russell]
     Full Idea: Frege showed that a cardinal number may be defined as a class of similar classes.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Bertrand Russell - Regressive Method for Premises in Mathematics p.277
Numbers are more than just 'second-level concepts', since existence is also one [Frege, by George/Velleman]
     Full Idea: Frege needs more than just saying that numbers are second-level concepts under which first-level concepts fall, because they can fall under many second-level concepts, such as that of existence.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by A.George / D.J.Velleman - Philosophies of Mathematics Ch.2
     A reaction: This marks the end of the first stage of Frege's theory, which leads him on to objects and Hume's Principle. After you have written 'level' a few times, you begin to wonder whether thought and world really are carved up in such a neat way.
"Number of x's such that ..x.." is a functional expression, yielding a name when completed [Frege, by George/Velleman]
     Full Idea: We can view "the number of x's such that ...x..." as a functional expression that is completed by a first-level predicate and yields a name (which is of the right kind to denote an object).
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by A.George / D.J.Velleman - Philosophies of Mathematics Ch.2
     A reaction: This is how Frege gets, in his account, from numbers being predicates to numbers being objects. He was a clever lad.
Frege gives an incoherent account of extensions resulting from abstraction [Fine,K on Frege]
     Full Idea: Frege identifies each conceptual abstract with the corresponding extension of concepts. But the extensions themselves are among the abstracts, so each extension is identical with the class of all concepts that have that extension, which is absurd.
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Kit Fine - The Limits of Abstraction I.2
     A reaction: Fine says this point is 'from the standpoint of a general theory of abstracts', which presumably was implied in Frege, but not actually spelled out.
For Frege the number of F's is a collection of first-level concepts [Frege, by George/Velleman]
     Full Idea: Frege defines 'the number of F's' as the extension of the concept 'equinumerous with F'. The extension of such a concept will be a collection of first-level concepts, namely, just those that are equinumerous with F.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by A.George / D.J.Velleman - Philosophies of Mathematics Ch.2
     A reaction: This must be reconciled with Frege's platonism, which tells us that numbers are objects, so the objects are second-level sets. Would there be third-level object/sets, such as the set of all the second-level sets perfectly divisible by three?
Numbers need to be objects, to define the extension of the concept of each successor to n [Frege, by George/Velleman]
     Full Idea: The fact that numbers are objects guarantees the availability of a supply of n+1 objects, which can be used to define the concept F for the successor of n, by defining the objects which fall under F.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by A.George / D.J.Velleman - Philosophies of Mathematics Ch.2
     A reaction: [compressed] This is the key step which takes from from numbers being adjectival to numbers being objectual. One wonders whether physical objects might do the necessary job at the next level down. Numbers need countables.
The number of F's is the extension of the second level concept 'is equipollent with F' [Frege, by Tait]
     Full Idea: Frege's definition is that the number N F(x) of F's, where F is a concept, is the extension of the second level concept 'is equipollent with F'.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by William W. Tait - Frege versus Cantor and Dedekind III
     A reaction: In trying to pin Frege down precisely, we must remember that an extension can be a collection of sets, as well as a collection of concrete particulars.
Frege showed that numbers attach to concepts, not to objects [Frege, by Wiggins]
     Full Idea: It was a justly celebrated insight of Frege that numbers attach to the concepts under which objects fall, and not to the objects themselves.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by David Wiggins - Sameness and Substance 1.6
     A reaction: A combination of this idea, and Aristotle's 'Categories', give us the roots of the philosophy of David Wiggins. Frege's example of two boots (or one 'pair' of boots) is the clearest example. …But the world dictates our concepts.
Frege replaced Cantor's sets as the objects of equinumerosity attributions with concepts [Frege, by Tait]
     Full Idea: Frege's contribution with respect to the definition of equinumerosity was to replace Cantor's sets as the objects of number attributions by concepts.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by William W. Tait - Frege versus Cantor and Dedekind XII
     A reaction: This pinpoints Frege's big idea, which is a powerful one, and may be right. The difficulty seems to be that the extension is ultimately what counts (because that is where plurality resides), and it is tricky getting the concept to determine the extension.
Zero is defined using 'is not self-identical', and one by using the concept of zero [Frege, by Weiner]
     Full Idea: Zero is the extension of 'is equinumerous with the concept "is not self-identical"' (which holds of no objects); ..one is defined as the extension of 'is equinumerous with the concept "is identical to zero"'.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Joan Weiner - Frege Ch.4
     A reaction: It sounds like some sort of cheating to define zero in terms of objects, but one in terms of concepts.
Frege said logical predication implies classes, which are arithmetical objects [Frege, by Morris,M]
     Full Idea: Frege's idea is that the logical notion of predication is enough to generate appropriate objects. Every predicate defines a class, which is in turn an object to which predicates apply; and the notion of a class can be used to generate arithmetic.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Morris - Guidebook to Wittgenstein's Tractatus 2H
     A reaction: At last, a lovely clear account of what Frege was doing - and why Russell's paradox was Frege's disaster. Logicism must take the ingredients of logic, and generate arithmetical 'objects' from them alone. But do we need 'objects'?
Frege started with contextual definition, but then switched to explicit extensional definition [Frege, by Wright,C]
     Full Idea: Frege abandoned contextual definition of numerical singular terms, and decided to go for explicit definitions in terms of extension-denoting terms instead.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Crispin Wright - Frege's Concept of Numbers as Objects 3.xiv
Each number, except 0, is the number of the concept of all of its predecessors [Frege, by Wright,C]
     Full Idea: In Frege's definition of numbers, each number, except 0, is defined as the number belonging to the concept under which just its predecessors fall.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Crispin Wright - Frege's Concept of Numbers as Objects 4.xvii
     A reaction: This would make the numbers dependent on all of the predecessors, just as Dedekind's numbers do. Dedekind's progression has to continue, but why should Frege's? Frege's are just there, where Dedekind's are constructed. Why are Frege's ordered?
Frege's account of cardinals fails in modern set theory, so they are now defined differently [Dummett on Frege]
     Full Idea: In standard set theory, Frege's cardinals could not be members of classes, and his proof of the infinity of natural numbers fails. Nowadays they are defined as sets each representative of its cardinality, comprising ordinals of lower cardinality.
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.14
     A reaction: Pinning something down in a unique way is not the same as telling you its intrinsic nature. But a completely successful definition seems to have locked on to some deep truth about its target.
Frege's incorrect view is that a number is an equivalence class [Benacerraf on Frege]
     Full Idea: Frege view (which has little to commend it) was that the number 3 is the extension of the concept 'equivalent with some 3-membered set'; that is, for Frege a number was an equivalence class - the class of all classes equivalent with a given class.
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Paul Benacerraf - What Numbers Could Not Be II
     A reaction: Frege is a platonist, who takes numbers to be objects, so this equivalence class must be identical with an object. What exactly was Frege claiming? I mean, really exactly?
The natural number n is the set of n-membered sets [Frege, by Yourgrau]
     Full Idea: Frege defines the natural number n in terms of the set of n-membered sets.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Palle Yourgrau - Sets, Aggregates and Numbers 'Two'
     A reaction: He says this view 'has been treated rudely by history', because Frege's view of sets was naive, and because independence results have undermined set-theoretic platonism.
A set doesn't have a fixed number, because the elements can be seen in different ways [Yourgrau on Frege]
     Full Idea: Given the set {Carter, Reagan} ...I still want to know How many what? Members? 2. Sets? 1. Feet of members? 4. Relatives of members? 44.
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Palle Yourgrau - Sets, Aggregates and Numbers 'New Problem'
     A reaction: This is his 'new problem' for Frege. Frege want a concept to divide a pack of cards, by cards, suits or pips. You choose 'pips' and form a set, but then the pips may have a number of corners. Solution: pick your 'objects' or 'units', and stick to it.
Frege's problem is explaining the particularity of numbers by general laws [Frege, by Burge]
     Full Idea: The worry with the attempt to derive arithmetic from general logical laws (which is required for it to be analytic apriori) is that it is incompatible with the particularity of numbers.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §13) by Tyler Burge - Frege on Apriority (with ps) 1
     A reaction: Burge cites §13 (end) of Grundlagen, and then the doomed Basic Law V as his attempt to bridge the gap from general to particular.
Individual numbers are best derived from the number one, and increase by one [Frege]
     Full Idea: The individual numbers are best derived from the number one together with increase by one.
     From: Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §18)
     A reaction: Frege rejects the empirical approach partly because of the intractability of zero, but this approach has the same problem. I suggest a combination of empiricism for simple numbers, and pure formalism for extensions into complexity, and zero.
'Exactly ten gallons' may not mean ten things instantiate 'gallon' [Rumfitt on Frege]
     Full Idea: To the question 'How many gallons of water are in the tank', the correct answer might be 'exactly ten'. But this does not mean that exactly ten things instantiate the concept 'gallon of water in the tank'.
     From: comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §46) by Ian Rumfitt - Concepts and Counting p.43
     A reaction: The difficulty for Frege that is being raised is that whole numbers are used to designate quantities of stuff, as well as for counting denumerable things. Rumfitt notes that 'ten' answers 'how much?' as well as Frege's 'how many?'.
A statement of number contains a predication about a concept [Frege]
     Full Idea: A statement of number [Zahlangabe] contains a predication about a concept.
     From: Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §46), quoted by Ian Rumfitt - Concepts and Counting Intro
     A reaction: See Rumfitt 'Concepts and Counting' for a discussion.
Numerical statements have first-order logical form, so must refer to objects [Frege, by Hodes]
     Full Idea: Summary: numerical terms are singular terms designating objects; numerical predicates are level 1 concepts and relations; quantification over mathematics is referential; hence arithmetic has first-order form, and mathematical objects exist, non-spatially.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §55?) by Harold Hodes - Logicism and Ontological Commits. of Arithmetic p.123
     A reaction: [compressed] So the heart of Frege is his translation of 'Jupiter has four moons' into a logical form which only refers to numerical objects. Commentators seem vague as to whether the theory is first-order or second-order.
The Number for F is the extension of 'equal to F' (or maybe just F itself) [Frege]
     Full Idea: My definition is as follows: the Number which belongs to the concept F is the extension of the concept 'equal to the concept F' [note: I believe that for 'extension of the concept' we could simply write 'concept'].
     From: Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §68)
     A reaction: The note has caused huge discussion [Maddy 1997:24]. No wonder I am confused about whether a Fregean number is a concept, or a property of a concept, or a collection of things, or an object. Or all four. Or none of the above.
Numbers are objects because they partake in identity statements [Frege, by Bostock]
     Full Idea: One can always say 'the number of Jupiter's moons is 4', which is explicitly a statement of identity, and for Frege identity is always to be construed as a relation between objects. This is really all he gives to argue that numbers are objects.
     From: report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], 55-57) by David Bostock - Philosophy of Mathematics
     A reaction: I struggle to understand why numbers turn out to be objects for Frege, given that they are defined in terms of sets of equinumerous sets. Is the number not a property of that meta-set. Bostock confirms my uncertainty. Paraphrase as solution?
In a number-statement, something is predicated of a concept [Frege]
     Full Idea: In a number-statement, something is predicated of a concept.
     From: Gottlob Frege (Review of Husserl's 'Phil of Arithmetic' [1894], p.328)
     A reaction: A succinct statement of Frege's theory of numbers. By my lights that would make numbers at least second-order abstractions.
If '5' is the set of all sets with five members, that may be circular, and you can know a priori if the set has content [Benardete,JA on Frege]
     Full Idea: There is a suspicion that Frege's definition of 5 (as the set of all sets with 5 members) may be infected with circularity, …and how can we be sure on a priori grounds that 4 and 5 are not both empty sets, and hence identical?
     From: comment on Gottlob Frege (works [1890]) by José A. Benardete - Metaphysics: the logical approach Ch.14
Numbers are properties of classes [Russell]
     Full Idea: Numbers are to be regarded as properties of classes.
     From: Bertrand Russell (The Principles of Mathematics [1903], §109)
     A reaction: If properties are then defined extensionally as classes, you end up with numbers as classes of classes.
Defining 'three' as the principle of collection or property of threes explains set theory definitions [Yourgrau]
     Full Idea: The Frege-Maddy definition of number (as the 'property' of being-three) explains why the definitions of Von Neumann, Zermelo and others work, by giving the 'principle of collection' that ties together all threes.
     From: Palle Yourgrau (Sets, Aggregates and Numbers [1985], 'A Fregean')
     A reaction: [compressed two or three sentences] I am strongly in favour of the best definition being the one which explains the target, rather than just pinning it down. I take this to be Aristotle's view.
Sameness of number is fundamental, not counting, despite children learning that first [Wright,C]
     Full Idea: We teach our children to count, sometimes with no attempt to explain what the sounds mean. Doubtless it is this habit which makes it so natural to think of the number series as fundamental. Frege's insight is that sameness of number is fundamental.
     From: Crispin Wright (Frege's Concept of Numbers as Objects [1983], 3.xv)
     A reaction: 'When do children understand number?' rather than when they can recite numerals. I can't make sense of someone being supposed to understand number without a grasp of which numbers are bigger or smaller. To make 13='15' do I add or subtract?
The existence of numbers is not a matter of identities, but of constituents of the world [Fine,K]
     Full Idea: On saying that a particular number exists, we are not saying that there is something identical to it, but saying something about its status as a genuine constituent of the world.
     From: Kit Fine (The Question of Ontology [2009], p.168)
     A reaction: This is aimed at Frege's criterion of identity, which is to be an element in an identity relation, such as x = y. Fine suggests that this only gives a 'trivial' notion of existence, when he is interested in a 'thick' sense of 'exists'.
The extension of concepts is not important to me [Maddy]
     Full Idea: I attach no decisive importance even to bringing in the extension of the concepts at all.
     From: Penelope Maddy (Naturalism in Mathematics [1997], §107)
     A reaction: He almost seems to equate the concept with its extension, but that seems to raise all sorts of questions, about indeterminate and fluctuating extensions.
In the ZFC hierarchy it is impossible to form Frege's set of all three-element sets [Maddy]
     Full Idea: In the ZFC cumulative hierarchy, Frege's candidates for numbers do not exist. For example, new three-element sets are formed at every stage, so there is no stage at which the set of all three-element sets could he formed.
     From: Penelope Maddy (Naturalism in Mathematics [1997], I.2)
     A reaction: Ah. This is a very important fact indeed if you are trying to understand contemporary discussions in philosophy of mathematics.
Numbers are universals, being sets whose instances are sets of appropriate cardinality [Lowe]
     Full Idea: My view is that numbers are universals, beings kinds of sets (that is, kinds whose particular instances are individual sets of appropriate cardinality).
     From: E.J. Lowe (The Possibility of Metaphysics [1998], 10)
     A reaction: [That is, 12 is the set of all sets which have 12 members] This would mean, I take it, that if the number of objects in existence was reduced to 11, 12 would cease to exist, which sounds wrong. Or are we allowed imagined instances?
A successor is the union of a set with its singleton [George/Velleman]
     Full Idea: For any set x, we define the 'successor' of x to be the set S(x) = x U {x}.
     From: A.George / D.J.Velleman (Philosophies of Mathematics [2002], Ch.3)
     A reaction: This is the Fregean approach to successor, where the Dedekind approach takes 'successor' to be a primitive. Frege 1884:§76.
Some 'how many?' answers are not predications of a concept, like 'how many gallons?' [Rumfitt]
     Full Idea: We hit trouble if we hear answers to some 'How many?' questions as predications about concepts. The correct answer to 'how many gallons of water are in the tank?' may be 'ten', but that doesn''t mean ten things instantiate 'gallon of water in the tank'.
     From: Ian Rumfitt (Concepts and Counting [2002], I)
     A reaction: Rumfitt makes the point that a huge number of things instantiate that concept in a ten gallon tank of water. No problem, says Rumfitt, because Frege wouldn't have counted that as a statement of number.