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Principia Mathematica
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{{For|Isaac Newton's book containing basic laws of physics|PhilosophiÃ¦ Naturalis Principia Mathematica}}{{distinguish|text=The Principles of Mathematicsâ€”another book of Russell published in 1903}}{{Italic title}}{{EngvarB|date=September 2013}}{{Use dmy dates|date=September 2013}}- the content below is remote from Wikipedia
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- Principia Mathematica 54-43.png -
âœ¸54.43: "From this proposition it will follow, when arithmetical addition has been defined, that 1 + 1 = 2." â€”Volume I, 1st edition, page 379 (page 362 in 2nd edition; page 360 in abridged version). (The proof is actually completed in Volume II, 1st edition, page 86, accompanied by the comment, "The above proposition is occasionally useful." Î¤hey go on to say "It is used at least three times, in âœ¸113.66 and âœ¸120.123.472.")
- Principia Mathematica 54-43.png -
âœ¸54.43: "From this proposition it will follow, when arithmetical addition has been defined, that 1 + 1 = 2." â€”Volume I, 1st edition, page 379 (page 362 in 2nd edition; page 360 in abridged version). (The proof is actually completed in Volume II, 1st edition, page 86, accompanied by the comment, "The above proposition is occasionally useful." Î¤hey go on to say "It is used at least three times, in âœ¸113.66 and âœ¸120.123.472.")
missing image!
- Russell, Whitehead - Principia Mathematica to 56.jpg -
The title page of the shortened Principia Mathematica to âœ¸56
}}}}The Principia Mathematica (often abbreviated PM) is a three-volume work on the foundations of mathematics written by Alfred North Whitehead and Bertrand Russell and published in 1910, 1912, and 1913. In 1925â€“27, it appeared in a second edition with an important Introduction to the Second Edition, an Appendix A that replaced âœ¸9 and all-new Appendix B and Appendix C.PM was an attempt to describe a set of axioms and inference rules in symbolic logic from which all mathematical truths could in principle be proven. As such, this ambitious project is of great importance in the history of mathematics and philosophy,WEB,weblink Principia Mathematica (Stanford Encyclopedia of Philosophy), Irvine, Andrew D., 1 May 2003, Metaphysics Research Lab, CSLI, Stanford University, 5 August 2009, being one of the foremost products of the belief that such an undertaking may be achievable. However, in 1931, GÃ¶del's incompleteness theorem proved definitively that PM, and in fact any other attempt, could never achieve this lofty goal; that is, for any set of axioms and inference rules proposed to encapsulate mathematics, either the system must be inconsistent, or there must in fact be some truths of mathematics which could not be deduced from them.One of the main inspirations and motivations for PM was the earlier work of Gottlob Frege on logic, which Russell discovered allowed for the construction of paradoxical sets. PM sought to avoid this problem by ruling out the unrestricted creation of arbitrary sets. This was achieved by replacing the notion of a general set with the notion of a hierarchy of sets of different 'types', a set of a certain type only allowed to contain sets of strictly lower types. Contemporary mathematics, however, avoids paradoxes such as Russell's in less unwieldy ways, such as the system of Zermeloâ€“Fraenkel set theory.PM is not to be confused with Russell's 1903 The Principles of Mathematics. PM states: "The present work was originally intended by us to be comprised in a second volume of Principles of Mathematics... But as we advanced, it became increasingly evident that the subject is a very much larger one than we had supposed; moreover on many fundamental questions which had been left obscure and doubtful in the former work, we have now arrived at what we believe to be satisfactory solutions."PM has long been known for its typographical complexity. Famously, several hundred pages of PM precede the proof of the validity of the proposition 1+1=2. The Modern Library placed it 23rd in a list of the top 100 English-language nonfiction books of the twentieth century.WEB,weblink The Modern Library's Top 100 Nonfiction Books of the Century, 30 April 1999, The New York Times Company, 5 August 2009, - Russell, Whitehead - Principia Mathematica to 56.jpg -
The title page of the shortened Principia Mathematica to âœ¸56
Scope of foundations laid
The Principia covered only set theory, cardinal numbers, ordinal numbers, and real numbers. Deeper theorems from real analysis were not included, but by the end of the third volume it was clear to experts that a large amount of known mathematics could in principle be developed in the adopted formalism. It was also clear how lengthy such a development would be.A fourth volume on the foundations of geometry had been planned, but the authors admitted to intellectual exhaustion upon completion of the third.Theoretical basis
As noted in the criticism of the theory by Kurt GÃ¶del (below), unlike a formalist theory, the "logicistic" theory of PM has no "precise statement of the syntax of the formalism". Another observation is that almost immediately in the theory, interpretations (in the sense of model theory) are presented in terms of truth-values for the behaviour of the symbols "âŠ¢" (assertion of truth), "~" (logical not), and "V" (logical inclusive OR).Truth-values: PM embeds the notions of "truth" and "falsity" in the notion "primitive proposition". A raw (pure) formalist theory would not provide the meaning of the symbols that form a "primitive proposition"â€”the symbols themselves could be absolutely arbitrary and unfamiliar. The theory would specify only how the symbols behave based on the grammar of the theory. Then later, by assignment of "values", a model would specify an interpretation of what the formulas are saying. Thus in the formal Kleene symbol set below, the "interpretation" of what the symbols commonly mean, and by implication how they end up being used, is given in parentheses, e.g., "Â¬ (not)". But this is not a pure Formalist theory.Contemporary construction of a formal theory
(File:Principia Mathematica List of Propositions.pdf|thumb|right|300px|list of propositions referred to by names)The following formalist theory is offered as contrast to the logicistic theory of PM. A contemporary formal system would be constructed as follows:- Symbols used: This set is the starting set, and other symbols can appear but only by definition from these beginning symbols. A starting set might be the following set derived from Kleene 1952: logical symbols: "â†’" (implies, IF-THEN, and "âŠƒ"), "&" (and), "V" (or), "Â¬" (not), "âˆ€" (for all), "âˆƒ" (there exists); predicate symbol "=" (equals); function symbols "+" (arithmetic addition), "âˆ™" (arithmetic multiplication), "'" (successor); individual symbol "0" (zero); variables "a", "b", "c", etc.; and parentheses "(" and ")".This set is taken from Kleene 1952:69 substituting â†’ for âŠƒ.
- Symbol strings: The theory will build "strings" of these symbols by concatenation (juxtaposition).Kleene 1952:71, Enderton 2001:15
- Formation rules: The theory specifies the rules of syntax (rules of grammar) usually as a recursive definition that starts with "0" and specifies how to build acceptable strings or "well-formed formulas" (wffs).Enderton 2001:16 This includes a rule for "substitution"This is the word used by Kleene 1952:78 of strings for the symbols called "variables" (as opposed to the other symbol-types).
- Transformation rule(s): The axioms that specify the behaviours of the symbols and symbol sequences.
- Rule of inference, detachment, modus ponens : The rule that allows the theory to "detach" a "conclusion" from the "premises" that led up to it, and thereafter to discard the "premises" (symbols to the left of the line â”‚, or symbols above the line if horizontal). If this were not the case, then substitution would result in longer and longer strings that have to be carried forward. Indeed, after the application of modus ponens, nothing is left but the conclusion, the rest disappears forever.
Contemporary theories often specify as their first axiom the classical or modus ponens or "the rule of detachment":
A, A âŠƒ B â”‚ B
The symbol "â”‚" is usually written as a horizontal line, here "âŠƒ" means "implies". The symbols A and B are "stand-ins" for strings; this form of notation is called an "axiom schema" (i.e., there is a countable number of specific forms the notation could take). This can be read in a manner similar to IF-THEN but with a difference: given symbol string IF A and A implies B THEN B (and retain only B for further use). But the symbols have no "interpretation" (e.g., no "truth table" or "truth values" or "truth functions") and modus ponens proceeds mechanistically, by grammar alone.
Construction
The theory of PM has both significant similarities, and similar differences, to a contemporary formal theory.{{clarify|date=October 2017|reason=What is the contemporary formal theory being referred to here?}} Kleene states that "this deduction of mathematics from logic was offered as intuitive axiomatics. The axioms were intended to be believed, or at least to be accepted as plausible hypotheses concerning the world".Quote from Kleene 1952:45. See discussion LOGICISM at pages 43â€“46. Indeed, unlike a Formalist theory that manipulates symbols according to rules of grammar, PM introduces the notion of "truth-values", i.e., truth and falsity in the real-world sense, and the "assertion of truth" almost immediately as the fifth and sixth elements in the structure of the theory (PM 1962:4â€“36):- Variables
- Uses of various letters
- The fundamental functions of propositions: "the Contradictory Function" symbolised by "~" and the "Logical Sum or Disjunctive Function" symbolised by "âˆ¨" being taken as primitive and logical implication defined (the following example also used to illustrate 9. Definition below) as
p âŠƒ q .=. ~ p âˆ¨ q Df. (PM 1962:11)
and logical product defined as
p . q .=. ~(~p âˆ¨ ~q) Df. (PM 1962:12)
- Equivalence: Logical equivalence, not arithmetic equivalence: "â‰¡" given as a demonstration of how the symbols are used, i.e., "Thus ' p â‰¡ q ' stands for '( p âŠƒ q ) . ( q âŠƒ p )'." (PM 1962:7). Notice that to discuss a notation PM identifies a "meta"-notation with "[space] ... [space]":In his section 8.5.4 Groping towards metalogic Grattan-Guinness 2000:454ff discusses the American logicians' critical reception of the second edition of PM. For instance Sheffer "puzzled that ' In order to give an account of logic, we must presuppose and employ logic ' " (p. 452). And Bernstein ended his 1926 review with the comment that "This distinction between the propositional logic as a mathematical system and as a language must be made, if serious errors are to be avoided; this distinction the Principia does not make" (p.454).
Logical equivalence appears again as a definition:
p â‰¡ q .=. ( p âŠƒ q ) . ( q âŠƒ p ) (PM 1962:12),
Notice the appearance of parentheses. This grammatical usage is not specified and appears sporadically; parentheses do play an important role in symbol strings, however, e.g., the notation "(x)" for the contemporary "âˆ€x".
- Truth-values: "The 'Truth-value' of a proposition is truth if it is true, and falsehood if it is false" (this phrase is due to Frege) (PM 1962:7).
- Assertion-sign: "'âŠ¦. p may be read 'it is true that' ... thus 'âŠ¦: p .âŠƒ. q ' means 'it is true that p implies q ', whereas 'âŠ¦. p .âŠƒâŠ¦.' q ' means ' p is true; therefore q is true'. The first of these does not necessarily involve the truth either of p or of q, while the second involves the truth of both" (PM 1962:92).
- Inference: PM 's version of modus ponens. "[If] 'âŠ¦. p ' and 'âŠ¦ (p âŠƒ q)' have occurred, then 'âŠ¦ . q ' will occur if it is desired to put it on record. The process of the inference cannot be reduced to symbols. Its sole record is the occurrence of 'âŠ¦. q ' [in other words, the symbols on the left disappear or can be erased]" (PM 1962:9).
- The use of dots
- Definitions: These use the "=" sign with "Df" at the right end.
- Summary of preceding statements: brief discussion of the primitive ideas "~ p" and "p âˆ¨ q" and "âŠ¦" prefixed to a proposition.
- Primitive propositions: the axioms or postulates. This was significantly modified in the 2nd edition.
- Propositional functions: The notion of "proposition" was significantly modified in the 2nd edition, including the introduction of "atomic" propositions linked by logical signs to form "molecular" propositions, and the use of substitution of molecular propositions into atomic or molecular propositions to create new expressions.
- The range of values and total variation
- Ambiguous assertion and the real variable: This and the next two sections were modified or abandoned in the 2nd edition. In particular, the distinction between the concepts defined in sections 15. Definition and the real variable and 16 Propositions connecting real and apparent variables was abandoned in the second edition.
- Formal implication and formal equivalence
- Identity
- Classes and relations
- Various descriptive functions of relations
- Plural descriptive functions
- Unit classes
Primitive ideas
Cf. PM 1962:90â€“94, for the first edition:- (1) Elementary propositions.
- (2) Elementary propositions of functions.
- (3) Assertion: introduces the notions of "truth" and "falsity".
- (4) Assertion of a propositional function.
- (5) Negation: "If p is any proposition, the proposition "not-p", or "p is false," will be represented by "~p" ".
- (6) Disjunction: "If p and q are any propositions, the proposition "p or q, i.e., "either p is true or q is true," where the alternatives are to be not mutually exclusive, will be represented by "p âˆ¨ q" ".
- (cf. section B)
Primitive propositions
The first edition (see discussion relative to the second edition, below) begins with a definition of the sign "âŠƒ"âœ¸1.01. p âŠƒ q .=. ~ p âˆ¨ q. Df.âœ¸1.1. Anything implied by a true elementary proposition is true. Pp modus ponens(âœ¸1.11 was abandoned in the second edition.)âœ¸1.2. âŠ¦: p âˆ¨ p .âŠƒ. p. Pp principle of tautologyâœ¸1.3. âŠ¦: q .âŠƒ. p âˆ¨ q. Pp principle of additionâœ¸1.4. âŠ¦: p âˆ¨ q .âŠƒ. q âˆ¨ p. Pp principle of permutationâœ¸1.5. âŠ¦: p âˆ¨ ( q âˆ¨ r ) .âŠƒ. q âˆ¨ ( p âˆ¨ r ). Pp associative principleâœ¸1.6. âŠ¦:. q âŠƒ r .âŠƒ: p âˆ¨ q .âŠƒ. p âˆ¨ r. Pp principle of summationâœ¸1.7. If p is an elementary proposition, ~p is an elementary proposition. Ppâœ¸1.71. If p and q are elementary propositions, p âˆ¨ q is an elementary proposition. Ppâœ¸1.72. If Ï†p and Ïˆp are elementary propositional functions which take elementary propositions as arguments, Ï†p âˆ¨ Ïˆp is an elementary proposition. PpTogether with the "Introduction to the Second Edition", the second edition's Appendix A abandons the entire section âœ¸9. This includes six primitive propositions âœ¸9 through âœ¸9.15 together with the Axioms of reducibility.The revised theory is made difficult by the introduction of the Sheffer stroke ("|") to symbolise "incompatibility" (i.e., if both elementary propositions p and q are true, their "stroke" p | q is false), the contemporary logical NAND (not-AND). In the revised theory, the Introduction presents the notion of "atomic proposition", a "datum" that "belongs to the philosophical part of logic". These have no parts that are propositions and do not contain the notions "all" or "some". For example: "this is red", or "this is earlier than that". Such things can exist ad finitum, i.e., even an "infinite enumeration" of them to replace "generality" (i.e., the notion of "for all").This idea is due to Wittgenstein's Tractatus. See the discussion at PM 1962:xivâ€“xv) PM then "advance[s] to molecular propositions" that are all linked by "the stroke". Definitions give equivalences for "~", "âˆ¨", "âŠƒ", and ".".The new introduction defines "elementary propositions" as atomic and molecular positions together. It then replaces all the primitive propositions âœ¸1.2 to âœ¸1.72 with a single primitive proposition framed in terms of the stroke:
"If p, q, r are elementary propositions, given p and p|(q|r), we can infer r. This is a primitive proposition."
The new introduction keeps the notation for "there exists" (now recast as "sometimes true") and "for all" (recast as "always true"). Appendix A strengthens the notion of "matrix" or "predicative function" (a "primitive idea", PM 1962:164) and presents four new Primitive propositions as âœ¸8.1â€“âœ¸8.13.âœ¸88. Multiplicative axiomâœ¸120. Axiom of infinityRamified types and the axiom of reducibility
In simple type theory objects are elements of various disjoint "types". Types are implicitly built up as follows. If Ï„1,...,Ï„m are types then there is a type (Ï„1,...,Ï„m) that can be thought of as the class of propositional functions of Ï„1,...,Ï„m (which in set theory is essentially the set of subsets of Ï„1Ã—...Ã—Ï„m). In particular there is a type () of propositions, and there may be a type Î¹ (iota) of "individuals" from which other types are built. Russell and Whitehead's notation for building up types from other types is rather cumbersome, and the notation here is due to Church.In the ramified type theory of PM all objects are elements of various disjoint ramified types. Ramified types are implicitly built up as follows. If Ï„1,...,Ï„m,Ïƒ1,...,Ïƒn are ramified types then as in simple type theory there is a type (Ï„1,...,Ï„m,Ïƒ1,...,Ïƒn) of "predicative" propositional functions of Ï„1,...,Ï„m,Ïƒ1,...,Ïƒn. However, there are also ramified types (Ï„1,...,Ï„m|Ïƒ1,...,Ïƒn) that can be thought of as the classes of propositional functions of Ï„1,...Ï„m obtained from propositional functions of type (Ï„1,...,Ï„m,Ïƒ1,...,Ïƒn) by quantifying over Ïƒ1,...,Ïƒn. When n=0 (so there are no Ïƒs) these propositional functions are called predicative functions or matrices. This can be confusing because current mathematical practice does not distinguish between predicative and non-predicative functions, and in any case PM never defines exactly what a "predicative function" actually is: this is taken as a primitive notion. Russell and Whitehead found it impossible to develop mathematics while maintaining the difference between predicative and non-predicative functions, so they introduced the axiom of reducibility, saying that for every non-predicative function there is a predicative function taking the same values. In practice this axiom essentially means that the elements of type (Ï„1,...,Ï„m|Ïƒ1,...,Ïƒn) can be identified with the elements of type (Ï„1,...,Ï„m), which causes the hierarchy of ramified types to collapse down to simple type theory. (Strictly speaking this is not quite correct, because PM allows two propositional functions to be different even if they take the same values on all arguments; this differs from current mathematical practice where one normally identifies two such functions.)In Zermelo set theory one can model the ramified type theory of PM as follows. One picks a set Î¹ to be the type of individuals. For example, Î¹ might be the set of natural numbers, or the set of atoms (in a set theory with atoms) or any other set one is interested in. Then if Ï„1,...,Ï„m are types, the type (Ï„1,...,Ï„m) is the power set of the product Ï„1Ã—...Ã—Ï„m, which can also be thought of informally as the set of (propositional predicative) functions from this product to a 2-element set {true,false}. The ramified type (Ï„1,...,Ï„m|Ïƒ1,...,Ïƒn) can be modeled as the product of the type (Ï„1,...,Ï„m,Ïƒ1,...,Ïƒn) with the set of sequences of n quantifiers (âˆ€ or âˆƒ) indicating which quantifier should be applied to each variable Ïƒi. (One can vary this slightly by allowing the Ïƒs to be quantified in any order, or allowing them to occur before some of the Ï„s, but this makes little difference except to the bookkeeping.)Notation
One author observes that "The notation in that work has been superseded by the subsequent development of logic during the 20th century, to the extent that the beginner has trouble reading PM at all"; while much of the symbolic content can be converted to modern notation, the original notation itself is "a subject of scholarly dispute", and some notation "embod[y] substantive logical doctrines so that it cannot simply be replaced by contemporary symbolism".BOOK,weblink The Stanford Encyclopedia of Philosophy, Bernard, Linsky, Edward N., Zalta, 1 May 2018, Metaphysics Research Lab, Stanford University, 1 May 2018, Stanford Encyclopedia of Philosophy, Kurt GÃ¶del was harshly critical of the notation:
"It is to be regretted that this first comprehensive and thorough-going presentation of a mathematical logic and the derivation of mathematics from it [is] so greatly lacking in formal precision in the foundations (contained in âœ¸1â€“âœ¸21 of Principia [i.e., sections âœ¸1â€“âœ¸5 (propositional logic), âœ¸8â€“14 (predicate logic with identity/equality), âœ¸20 (introduction to set theory), and âœ¸21 (introduction to relations theory)]) that it represents in this respect a considerable step backwards as compared with Frege. What is missing, above all, is a precise statement of the syntax of the formalism. Syntactical considerations are omitted even in cases where they are necessary for the cogency of the proofs".Kurt GÃ¶del 1944 "Russell's mathematical logic" appearing at page 120 in Feferman et al. 1990 Kurt GÃ¶del Collected Works Volume II, Oxford University Press, NY, {{isbn|978-0-19-514721-6}}(v.2.pbk.) .
This is reflected in the example below of the symbols "p", "q", "r" and "âŠƒ" that can be formed into the string "p âŠƒ q âŠƒ r". PM requires a definition of what this symbol-string means in terms of other symbols; in contemporary treatments the "formation rules" (syntactical rules leading to "well formed formulas") would have prevented the formation of this string.Source of the notation: Chapter I "Preliminary Explanations of Ideas and Notations" begins with the source of the elementary parts of the notation (the symbols =âŠƒâ‰¡âˆ’Î›VÎµ and the system of dots):
"The notation adopted in the present work is based upon that of Peano, and the following explanations are to some extent modeled on those which he prefixes to his Formulario Mathematico [i.e., Peano 1889]. His use of dots as brackets is adopted, and so are many of his symbols" (PM 1927:4).For comparison, see the translated portion of Peano 1889 in van Heijenoort 1967:81ff.
PM changed Peano's Æ† to âŠƒ, and also adopted a few of Peano's later symbols, such as â„© and Î¹, and Peano's practice of turning letters upside down.PM adopts the assertion sign "âŠ¦" from Frege's 1879 Begriffsschrift:This work can be found at van Heijenoort 1967:1ff.
"(I)t may be read 'it is true that'"And see footnote, both at PM 1927:92
Thus to assert a proposition p PM writes:
"âŠ¦. p." (PM 1927:92)
(Observe that, as in the original, the left dot is square and of greater size than the period on the right.)Most of the rest of the notation in PM was invented by Whitehead. {{Citation needed|date=March 2017}}An introduction to the notation of "Section A Mathematical Logic" (formulas âœ¸1â€“âœ¸5.71)
âœ¸3.12. âŠ¢ : ~p . v . ~q . v . p . q
corresponds to
(((~p) v (~q)) v (p âˆ§ q))
where the colon represents the outer (), the next two dots represent the parentheses around ~p and ~q, the third dot represents the parentheses around p âˆ§ q, and the fourth dot (rather confusingly) represents the logical symbol âˆ§ rather than a pair of parentheses. This uses the definition (followed by the explanatory comment):
âœ¸2.33 p v q v r .=. (p v q) v r Df
This definition serves only for the avoidance of brackets.
âœ¸9.521. âŠ¢ :â€‰: (âˆƒx). Ï†x . âŠƒ . q : âŠƒ :â€‰. (âˆƒx). Ï†x . v . r : âŠƒ . q v r
stands for
((((âˆƒx)(Ï†x)) âŠƒ (q)) âŠƒ ((((âˆƒx) (Ï†x)) v (r)) âŠƒ (q v r)))
Example 3, with a double dot indicating a logical symbol (from volume 1, page 10):
pâŠƒq:qâŠƒr.âŠƒ.pâŠƒr
stands for
((pâŠƒq) âˆ§ (qâŠƒr))âŠƒ(pâŠƒr)
where the double dot represents the logical symbol âˆ§ and can be viewed as having the same priority as a non-logical single dot.Later in section âœ¸14, brackets "[ ]" appear, and in sections âœ¸20 and following, braces "{ }" appear. Whether these symbols have specific meanings or are just for visual clarification is unclear. Unfortunately the single dot (but also ":", ":.", "::", etc.) is also used to symbolise "logical product" (contemporary logical AND often symbolised by "&" or "âˆ§").Logical implication is represented by Peano's "Æ†" simplified to "âŠƒ", logical negation is symbolised by an elongated tilde, i.e., "~" (contemporary "~" or "Â¬"), the logical OR by "v". The symbol "=" together with "Df" is used to indicate "is defined as", whereas in sections âœ¸13 and following, "=" is defined as (mathematically) "identical with", i.e., contemporary mathematical "equality" (cf. discussion in section âœ¸13). Logical equivalence is represented by "â‰¡" (contemporary "if and only if"); "elementary" propositional functions are written in the customary way, e.g., "f(p)", but later the function sign appears directly before the variable without parenthesis e.g., "Ï†x", "Ï‡x", etc.Example, PM introduces the definition of "logical product" as follows:
âœ¸3.01. p . q .=. ~(~p v ~q) Df.
Translation of the formulas into contemporary symbols: Various authors use alternate symbols, so no definitive translation can be given. However, because of criticisms such as that of Kurt GÃ¶del below, the best contemporary treatments will be very precise with respect to the "formation rules" (the syntax) of the formulas.The first formula might be converted into modern symbolism as follows:The first example comes from plato.stanford.edu (loc.cit.).
where "p . q" is the logical product of p and q.
âœ¸3.02. p âŠƒ q âŠƒ r .=. p âŠƒ q . q âŠƒ r Df.
This definition serves merely to abbreviate proofs.
(p & q) =df (~(~p v ~q))
alternately
(p & q) =df (Â¬(Â¬p v Â¬q))
alternately
(p âˆ§ q) =df (Â¬(Â¬p v Â¬q))
etc.The second formula might be converted as follows:
(p â†’ q â†’ r) =df (p â†’ q) & (q â†’ r)
But note that this is not (logically) equivalent to (p â†’ (q â†’ r)) nor to ((p â†’ q) â†’ r), and these two are not logically equivalent either.An introduction to the notation of "Section B Theory of Apparent Variables" (formulas âœ¸8â€“âœ¸14.34)
These sections concern what is now known as predicate logic, and predicate logic with identity (equality).
* NB: As a result of criticism and advances, the second edition of PM (1927) replaces âœ¸9 with a new âœ¸8 (Appendix A). This new section eliminates the first edition's distinction between real and apparent variables, and it eliminates "the primitive idea 'assertion of a propositional function'.page xiii of 1927 appearing in the 1962 paperback edition to âœ¸56. To add to the complexity of the treatment, âœ¸8 introduces the notion of substituting a "matrix", and the Sheffer stroke:
Section âœ¸10: The existential and universal "operators": PM adds "(x)" to represent the contemporary symbolism "for all x " i.e., " âˆ€x", and it uses a backwards serifed E to represent "there exists an x", i.e., "(ÆŽx)", i.e., the contemporary "âˆƒx". The typical notation would be similar to the following:
* Matrix: In contemporary usage, PM 's matrix is (at least for propositional functions), a truth table, i.e., all truth-values of a propositional or predicate function.
* Sheffer stroke: Is the contemporary logical NAND (NOT-AND), i.e., "incompatibility", meaning:
"Given two propositions p and q, then ' p | q ' means "proposition p is incompatible with proposition q", i.e., if both propositions p and q evaluate as true, then and only then p | q evaluates as false." After section âœ¸8 the Sheffer stroke sees no usage.
"(x) . Ï†x" means "for all values of variable x, function Ï† evaluates to true"
"(ÆŽx) . Ï†x" means "for some value of variable x, function Ï† evaluates to true"
Sections âœ¸10, âœ¸11, âœ¸12: Properties of a variable extended to all individuals: section âœ¸10 introduces the notion of "a property" of a "variable". PM gives the example: Ï† is a function that indicates "is a Greek", and Ïˆ indicates "is a man", and Ï‡ indicates "is a mortal" these functions then apply to a variable x. PM can now write, and evaluate:
(x) . Ïˆx
The notation above means "for all x, x is a man". Given a collection of individuals, one can evaluate the above formula for truth or falsity. For example, given the restricted collection of individuals { Socrates, Plato, Russell, Zeus } the above evaluates to "true" if we allow for Zeus to be a man. But it fails for:
(x) . Ï†x
because Russell is not Greek. And it fails for
(x) . Ï‡x
because Zeus is not a mortal.Equipped with this notation PM can create formulas to express the following: "If all Greeks are men and if all men are mortals then all Greeks are mortals". (PM 1962:138)
(x) . Ï†x âŠƒ Ïˆx :(x). Ïˆx âŠƒ Ï‡x :âŠƒ: (x) . Ï†x âŠƒ Ï‡x
Another example: the formula:
âœ¸10.01. (ÆŽx). Ï†x . = . ~(x) . ~Ï†x Df.
means "The symbols representing the assertion 'There exists at least one x that satisfies function Ï†' is defined by the symbols representing the assertion 'It's not true that, given all values of x, there are no values of x satisfying Ï†'".The symbolisms âŠƒx and "â‰¡x" appear at âœ¸10.02 and âœ¸10.03. Both are abbreviations for universality (i.e., for all) that bind the variable x to the logical operator. Contemporary notation would have simply used parentheses outside of the equality ("=") sign:
âœ¸10.02 Ï†x âŠƒx Ïˆx .=. (x). Ï†x âŠƒ Ïˆx Df
PM attributes the first symbolism to Peano.Section âœ¸11 applies this symbolism to two variables. Thus the following notations: âŠƒx, âŠƒy, âŠƒx, y could all appear in a single formula.Section âœ¸12 reintroduces the notion of "matrix" (contemporary truth table), the notion of logical types, and in particular the notions of first-order and second-order functions and propositions.New symbolism "Ï† ! x" represents any value of a first-order function. If a circumflex "ï¼¾" is placed over a variable, then this is an "individual" value of y, meaning that "Å·" indicates "individuals" (e.g., a row in a truth table); this distinction is necessary because of the matrix/extensional nature of propositional functions.Now equipped with the matrix notion, PM can assert its controversial axiom of reducibility: a function of one or two variables (two being sufficient for PM 's use) where all its values are given (i.e., in its matrix) is (logically) equivalent ("â‰¡") to some "predicative" function of the same variables. The one-variable definition is given below as an illustration of the notation (PM 1962:166â€“167):âœ¸12.1 âŠ¢: (ÆŽ f): Ï†x .â‰¡x. f ! x Pp;
Contemporary notation: âˆ€x(Ï†(x) â†’ Ïˆ(x)) (or a variant)
âœ¸10.03 Ï†x â‰¡x Ïˆx .=. (x). Ï†x â‰¡ Ïˆx Df
Contemporary notation: âˆ€x(Ï†(x) â†” Ïˆ(x)) (or a variant)
Pp is a "Primitive proposition" ("Propositions assumed without proof") (PM 1962:12, i.e., contemporary "axioms"), adding to the 7 defined in section âœ¸1 (starting with âœ¸1.1 modus ponens). These are to be distinguished from the "primitive ideas" that include the assertion sign "âŠ¢", negation "~", logical OR "V", the notions of "elementary proposition" and "elementary propositional function"; these are as close as PM comes to rules of notational formation, i.e., syntax.
âœ¸13.01. x = y .=: (Ï†): Ï† ! x . âŠƒ . Ï† ! y Df
means:
"This definition states that x and y are to be called identical when every predicative function satisfied by x is also satisfied by y ... Note that the second sign of equality in the above definition is combined with "Df", and thus is not really the same symbol as the sign of equality which is defined."
The not-equals sign "â‰ " makes its appearance as a definition at âœ¸13.02.âœ¸14: Descriptions:
"A description is a phrase of the form "the term y which satisfies Ï†Å·, where Ï†Å· is some function satisfied by one and only one argument."The original typography employs an x with a circumflex rather than Å·; this continues below
From this PM employs two new symbols, a forward "E" and an inverted iota "â„©". Here is an example:
âœ¸14.02. E ! ( â„©y) (Ï†y) .=: ( ÆŽb):Ï†y . â‰¡y . y = b Df.
This has the meaning:
"The y satisfying Ï†Å· exists," which holds when, and only when Ï†Å· is satisfied by one value of y and by no other value." (PM 1967:173â€“174)
Introduction to the notation of the theory of classes and relations
The text leaps from section âœ¸14 directly to the foundational sections âœ¸20 GENERAL THEORY OF CLASSES and âœ¸21 GENERAL THEORY OF RELATIONS. "Relations" are what is known in contemporary set theory as sets of ordered pairs. Sections âœ¸20 and âœ¸22 introduce many of the symbols still in contemporary usage. These include the symbols "Îµ", "âŠ‚", "âˆ©", "âˆª", "â€“", "Î›", and "V": "Îµ" signifies "is an element of" (PM 1962:188); "âŠ‚" (âœ¸22.01) signifies "is contained in", "is a subset of"; "âˆ©" (âœ¸22.02) signifies the intersection (logical product) of classes (sets); "âˆª" (âœ¸22.03) signifies the union (logical sum) of classes (sets); "â€“" (âœ¸22.03) signifies negation of a class (set); "Î›" signifies the null class; and "V" signifies the universal class or universe of discourse.Small Greek letters (other than "Îµ", "Î¹", "Ï€", "Ï†", "Ïˆ", "Ï‡", and "Î¸") represent classes (e.g., "Î±", "Î²", "Î³", "Î´", etc.) (PM 1962:188):
x Îµ Î±
When applied to relations in section âœ¸23 CALCULUS OF RELATIONS, the symbols "âŠ‚", "âˆ©", "âˆª", and "â€“" acquire a dot: for example: "âŠ", "âˆ¸".The "âŠ‚" sign has a dot inside it, and the intersection sign "âˆ©" has a dot above it; these are not available in the "Arial Unicode MS" font.The notion, and notation, of "a class" (set): In the first edition PM asserts that no new primitive ideas are necessary to define what is meant by "a class", and only two new "primitive propositions" called the axioms of reducibility for classes and relations respectively (PM 1962:25).Wiener 1914 "A simplification of the logic of relations" (van Heijenoort 1967:224ff) disposed of the second of these when he showed how to reduce the theory of relations to that of classes But before this notion can be defined, PM feels it necessary to create a peculiar notation "áº‘(Ï†z)" that it calls a "fictitious object". (PM 1962:188)
"The use of single letter in place of symbols such as áº‘(Ï†z) or áº‘(Ï† ! z) is practically almost indispensable, since otherwise the notation rapidly becomes intolerably cumbrous. Thus ' x Îµ Î±' will mean ' x is a member of the class Î±'". (PM 1962:188)
Î± âˆª â€“Î± = V
The union of a set and its inverse is the universal (completed) set.See the ten postulates of Huntington, in particular postulates IIa and IIb at PM 1962:205 and discussion at page 206.
Î± âˆ© â€“Î± = Î›
The intersection of a set and its inverse is the null (empty) set.
âŠ¢: x Îµ áº‘(Ï†z) .â‰¡. (Ï†x)
At least PM can tell the reader how these fictitious objects behave, because "A class is wholly determinate when its membership is known, that is, there cannot be two different classes having the same membership" (PM 1962:26). This is symbolised by the following equality (similar to âœ¸13.01 above:
"i.e., ' x is a member of the class determined by (Ï†áº‘)' is [logically] equivalent to ' x satisfies (Ï†áº‘),' or to '(Ï†x) is true.'". (PM 1962:25)
áº‘(Ï†z) = áº‘(Ïˆz) . â‰¡ : (x): Ï†x .â‰¡. Ïˆx
Perhaps the above can be made clearer by the discussion of classes in Introduction to the 2nd Edition, which disposes of the Axiom of Reducibility and replaces it with the notion: "All functions of functions are extensional" (PM 1962:xxxix), i.e.,
"This last is the distinguishing characteristic of classes, and justifies us in treating áº‘(Ïˆz) as the class determined by [the function] Ïˆáº‘." (PM 1962:188)
Ï†x â‰¡x Ïˆx .âŠƒ. (x): Æ’(Ï†áº‘) â‰¡ Æ’(Ïˆáº‘) (PM 1962:xxxix)
This has the reasonable meaning that "IF for all values of x the truth-values of the functions Ï† and Ïˆ of x are [logically] equivalent, THEN the function Æ’ of a given Ï†áº‘ and Æ’ of Ïˆáº‘ are [logically] equivalent." PM asserts this is "obvious":
"This is obvious, since Ï† can only occur in Æ’(Ï†áº‘) by the substitution of values of Ï† for p, q, r, ... in a [logical-] function, and, if Ï†x â‰¡ Ïˆx, the substitution of Ï†x for p in a [logical-] function gives the same truth-value to the truth-function as the substitution of Ïˆx. Consequently there is no longer any reason to distinguish between functions classes, for we have, in virtue of the above,
Ï†x â‰¡x Ïˆx .âŠƒ. (x). Ï†áº‘ = . Ïˆáº‘".
Observe the change to the equality "=" sign on the right. PM goes on to state that will continue to hang onto the notation "áº‘(Ï†z)", but this is merely equivalent to Ï†áº‘, and this is a class. (all quotes: PM 1962:xxxix).Consistency and criticisms
According to Carnap's "Logicist Foundations of Mathematics", Russell wanted a theory that could plausibly be said to derive all of mathematics from purely logical axioms. However, Principia Mathematica required, in addition to the basic axioms of type theory, three further axioms that seemed to not be true as mere matters of logic, namely the axiom of infinity, the axiom of choice, and the axiom of reducibility. Since the first two were existential axioms, Russell phrased mathematical statements depending on them as conditionals. But reducibility was required to be sure that the formal statements even properly express statements of real analysis, so that statements depending on it could not be reformulated as conditionals. Frank P. Ramsey tried to argue that Russell's ramification of the theory of types was unnecessary, so that reducibility could be removed, but these arguments seemed inconclusive.Beyond the status of the axioms as logical truths, one can ask the following questions about any system such as PM:- whether a contradiction could be derived from the axioms (the question of inconsistency), and
- whether there exists a mathematical statement which could neither be proven nor disproven in the system (the question of completeness).
GÃ¶del 1930, 1931
In 1930, GÃ¶del's completeness theorem showed that first-order predicate logic itself was complete in a much weaker senseâ€”that is, any sentence that is unprovable from a given set of axioms must actually be false in some model of the axioms. However, this is not the stronger sense of completeness desired for Principia Mathematica, since a given system of axioms (such as those of Principia Mathematica) may have many models, in some of which a given statement is true and in others of which that statement is false, so that the statement is left undecided by the axioms.GÃ¶del's incompleteness theorems cast unexpected light on these two related questions.GÃ¶del's first incompleteness theorem showed that no recursive extension of Principia could be both consistent and complete for arithmetic statements. (As mentioned above, Principia itself was already known to be incomplete for some non-arithmetic statements.) According to the theorem, within every sufficiently powerful recursive logical system (such as Principia), there exists a statement G that essentially reads, "The statement G cannot be proved." Such a statement is a sort of Catch-22: if G is provable, then it is false, and the system is therefore inconsistent; and if G is not provable, then it is true, and the system is therefore incomplete.GÃ¶del's second incompleteness theorem (1931) shows that no formal system extending basic arithmetic can be used to prove its own consistency. Thus, the statement "there are no contradictions in the Principia system" cannot be proven in the Principia system unless there are contradictions in the system (in which case it can be proven both true and false).Wittgenstein 1919, 1939
By the second edition of PM, Russell had removed his axiom of reducibility to a new axiom (although he does not state it as such). GÃ¶del 1944:126 describes it this way:
"This change is connected with the new axiom that functions can occur in propositions only "through their values", i.e., extensionally . . . [this is] quite unobjectionable even from the constructive standpoint . . . provided that quantifiers are always restricted to definite orders". This change from a quasi-intensional stance to a fully extensional stance also restricts predicate logic to the second order, i.e. functions of functions: "We can decide that mathematics is to confine itself to functions of functions which obey the above assumption" (PM 2nd Edition p. 401, Appendix C).
This new proposal resulted in a dire outcome. An "extensional stance" and restriction to a second-order predicate logic means that a propositional function extended to all individuals such as "All 'x' are blue" now has to list all of the 'x' that satisfy (are true in) the proposition, listing them in a possibly infinite conjunction: e.g. x1 âˆ§ x2 âˆ§ . . . âˆ§ xn âˆ§ . . .. Ironically, this change came about as the result of criticism from Wittgenstein in his 1919 Tractatus Logico-Philosophicus. As described by Russell in the Preface to the 2nd edition of PM:
"There is another course, recommended by Wittgensteinâ€ (â€ Tractatus Logico-Philosophicus, *5.54ff) for philosophical reasons. This is to assume that functions of propositions are always truth-functions, and that a function can only occur in a proposition through its values. . . . [Working through the consequences] it appears that everything in Vol. I remains true . . . the theory of inductive cardinals and ordinals survives; but it seems that the theory of infinite Dedekindian and well-ordered series largely collapses, so that irrationals, and real numbers generally, can no longer be adequately dealt with. Also Cantor's proof that 2n > n breaks down unless n is finite." (PM 2nd edition reprinted 1962:xiv, also cf new Appendix C).
In other words, the fact that an infinite list cannot realistically be specified means that the concept of "number" in the infinite sense (i.e. the continuum) cannot be described by the new theory proposed in PM Second Edition.Wittgenstein in his Lectures on the Foundations of Mathematics, Cambridge 1939 criticised Principia on various grounds, such as: - It purports to reveal the fundamental basis for arithmetic. However, it is our everyday arithmetical practices such as counting which are fundamental; for if a persistent discrepancy arose between counting and Principia, this would be treated as evidence of an error in Principia (e.g., that Principia did not characterise numbers or addition correctly), not as evidence of an error in everyday counting.
- The calculating methods in Principia can only be used in practice with very small numbers. To calculate using large numbers (e.g., billions), the formulae would become too long, and some short-cut method would have to be used, which would no doubt rely on everyday techniques such as counting (or else on non-fundamental and hence questionable methods such as induction). So again Principia depends on everyday techniques, not vice versa.
GÃ¶del 1944
In his 1944 Russell's mathematical logic, GÃ¶del offers a "critical but sympathetic discussion of the logicistic order of ideas":Kleene 1952:46.
"It is to be regretted that this first comprehensive and thorough-going presentation of a mathematical logic and the derivation of mathematics from it [is] so greatly lacking in formal precision in the foundations (contained in *1-*21 of Principia) that it represents in this respect a considerable step backwards as compared with Frege. What is missing, above all, is a precise statement of the syntax of the formalism. Syntactical considerations are omitted even in cases where they are necessary for the cogency of the proofs . . . The matter is especially doubtful for the rule of substitution and of replacing defined symbols by their definiens . . . it is chiefly the rule of substitution which would have to be proved" (GÃ¶del 1944:124)GÃ¶del 1944 Russell's mathematical logic in Kurt GÃ¶del: Collected Works Volume II, Oxford University Press, New York, NY, {{isbn|978-0-19-514721-6}}.
Contents
Part I Mathematical logic. Volume I âœ¸1 to âœ¸43
This section describes the propositional and predicate calculus, and gives the basic properties of classes, relations, and types.Part II Prolegomena to cardinal arithmetic. Volume I âœ¸50 to âœ¸97
This part covers various properties of relations, especially those needed for cardinal arithmetic.Part III Cardinal arithmetic. Volume II âœ¸100 to âœ¸126
This covers the definition and basic properties of cardinals. A cardinal is defined to be an equivalence class of similar classes (as opposed to ZFC, where a cardinal is a special sort of von Neumann ordinal). Each type has its own collection of cardinals associated with it, and there is a considerable amount of bookkeeping necessary for comparing cardinals of different types. PM define addition, multiplication and exponentiation of cardinals, and compare different definitions of finite and infinite cardinals. âœ¸120.03 is the Axiom of infinity.Part IV Relation-arithmetic. Volume II âœ¸150 to âœ¸186
A "relation-number" is an equivalence class of isomorphic relations. PM defines analogues of addition, multiplication, and exponentiation for arbitrary relations. The addition and multiplication is similar to the usual definition of addition and multiplication of ordinals in ZFC, though the definition of exponentiation of relations in PM is not equivalent to the usual one used in ZFC.Part V Series. Volume II âœ¸200 to âœ¸234 and volume III âœ¸250 to âœ¸276
This covers series, which is PM's term for what is now called a totally ordered set. In particular it covers complete series, continuous functions between series with the order topology (though of course they do not use this terminology), well-ordered series, and series without "gaps" (those with a member strictly between any two given members).Part VI Quantity. Volume III âœ¸300 to âœ¸375
This section constructs the ring of integers, the fields of rational and real numbers, and "vector-families", which are related to what are now called torsors over abelian groups.Comparison with set theory
This section compares the system in PM with the usual mathematical foundations of ZFC. The system of PM is roughly comparable in strength with Zermelo set theory (or more precisely a version of it where the axiom of separation has all quantifiers bounded).- The system of propositional logic and predicate calculus in PM is essentially the same as that used now, except that the notation and terminology has changed.
- The most obvious difference between PM and set theory is that in PM all objects belong to one of a number of disjoint types. This means that everything gets duplicated for each (infinite) type: for example, each type has its own ordinals, cardinals, real numbers, and so on. This results in a lot of bookkeeping to relate the various types with each other.
- In ZFC functions are normally coded as sets of ordered pairs. In PM functions are treated rather differently. First of all, "function" means "propositional function", something taking values true or false. Second, functions are not determined by their values: it is possible to have several different functions all taking the same values (for example, one might regard 2x+2 and 2(x+1) as different functions on grounds that the computer programs for evaluating them are different). The functions in ZFC given by sets of ordered pairs correspond to what PM call "matrices", and the more general functions in PM are coded by quantifying over some variables. In particular PM distinguishes between functions defined using quantification and functions not defined using quantification, whereas ZFC does not make this distinction.
- PM has no analogue of the axiom of replacement, though this is of little practical importance as this axiom is used very little in mathematics outside set theory.
- PM emphasizes relations as a fundamental concept, whereas in current mathematical practice it is functions rather than relations that are treated as more fundamental; for example, category theory emphasizes morphisms or functions rather than relations. (However, there is an analogue of categories called allegories that models relations rather than functions, and is quite similar to the type system of PM.)
- In PM, cardinals are defined as classes of similar classes, whereas in ZFC cardinals are special ordinals. In PM there is a different collection of cardinals for each type with some complicated machinery for moving cardinals between types, whereas in ZFC there is only 1 sort of cardinal. Since PM does not have any equivalent of the axiom of replacement, it is unable to prove the existence of cardinals greater than â„µÏ‰.
- In PM ordinals are treated as equivalence classes of well-ordered sets, and as with cardinals there is a different collection of ordinals for each type. In ZFC there is only one collection of ordinals, usually defined as von Neumann ordinals. One strange quirk of PM is that they do not have an ordinal corresponding to 1, which causes numerous unnecessary complications in their theorems. The definition of ordinal exponentiation Î±Î² in PM is not equivalent to the usual definition in ZFC and has some rather undesirable properties: for example, it is not continuous in Î² and is not well ordered (so is not even an ordinal).
- The constructions of the integers, rationals and real numbers in ZFC have been streamlined considerably over time since the constructions in PM.
Differences between editions
Apart from corrections of misprints, the main text of PM is unchanged between the first and second editions. The main text in volumes 1 and 2 was reset, so that it occupies fewer pages in each. In the second edition volume 3 was not reset, being photographically reprinted with the same page numbering; corrections were still made. The total number of pages (excluding the endpapers) in the first edition is 1,996; in the second, 2,000. Volume 1 has five new additions:- A 54-page introduction by Russell describing the changes they would have made had they had more time and energy. The main change he suggests is the removal of the controversial axiom of reducibility, though he admits that he knows no satisfactory substitute for it. He also seems more favorable to the idea that a function should be determined by its values (as is usual in current mathematical practice).
- Appendix A, numbered as 8, 15 pages about the Sheffer stroke.
- Appendix B, numbered as 89, discussing induction without the axiom of reducibility
- Appendix C, 8 pages discussing propositional functions
- An 8-page list of definitions at the end, giving a much-needed index to the 500 or so notations used.
See also
- Axiomatic set theory
- Begriffsschrift
- Boolean algebra (logic)
- Information Processing Language â€“ first computational demonstration of theorems in PM
Footnotes
{{reflist|35em}}References
Primary:- {{citation
- {{citation
- {{citation
- {{citation
- {{citation
- {{citation
- {{citation|mr=1700771
- Stephen Kleene 1952 Introduction to Meta-Mathematics, 6th Reprint, North-Holland Publishing Company, Amsterdam NY, {{isbn|0-7204-2103-9}}.
- BOOK, Stephen Cole Kleene, Stephen Cole Kleene, Michael Beeson, March 2009, Introduction to Metamathematics, Paperback, Ishi Press, 978-0-923891-57-2, Kleene2009,
- Ivor Grattan-Guinness (2000) The Search for Mathematical Roots 1870â€“1940, Princeton University Press, Princeton N.J., {{isbn|0-691-05857-1}} (alk. paper).
- Ludwig Wittgenstein 2009 Major Works: Selected Philosophical Writings, HarperrCollins, NY, NY, {{isbn|978-0-06-155024-9}}. In particular:
Tractatus Logico-Philosophicus (Vienna 1918, original publication in German).
- Jean van Heijenoort editor 1967 From Frege to GÃ¶del: A Source book in Mathematical Logic, 1879â€“1931, 3rd printing, Harvard University Press, Cambridge MA, {{isbn|0-674-32449-8}} (pbk.)
- Michel Weber and Will Desmond (eds.). Handbook of Whiteheadian Process Thought, Frankfurt / Lancaster, Ontos Verlag, Process Thought X1 & X2, 2008.
External links
- Stanford Encyclopedia of Philosophy:
- Principia Mathematicaâ€”by A. D. Irvine.
- The Notation in Principia Mathematicaâ€”by Bernard Linsky.
- Proposition âœ¸54.43 in a more modern notation (Metamath)
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