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Theorem List for Metamath Proof Explorer - 2301-2400   *Has distinct variable group(s)
TypeLabelDescription
Statement
 
Theoremmoimi 2301 "At most one" is preserved through implication (notice wff reversal). (Contributed by NM, 15-Feb-2006.)
 |-  ( ph  ->  ps )   =>    |-  ( E* x ps  ->  E* x ph )
 
Theoremmorimv 2302* Move antecedent outside of "at most one." (Contributed by NM, 28-Jul-1995.)
 |-  ( E* x (
 ph  ->  ps )  ->  ( ph  ->  E* x ps )
 )
 
Theoremeuimmo 2303 Uniqueness implies "at most one" through implication. (Contributed by NM, 22-Apr-1995.)
 |-  ( A. x (
 ph  ->  ps )  ->  ( E! x ps  ->  E* x ph ) )
 
Theoremeuim 2304 Add existential uniqueness quantifiers to an implication. Note the reversed implication in the antecedent. (Contributed by NM, 19-Oct-2005.) (Proof shortened by Andrew Salmon, 14-Jun-2011.)
 |-  ( ( E. x ph 
 /\  A. x ( ph  ->  ps ) )  ->  ( E! x ps  ->  E! x ph ) )
 
Theoremmoan 2305 "At most one" is still the case when a conjunct is added. (Contributed by NM, 22-Apr-1995.)
 |-  ( E* x ph  ->  E* x ( ps 
 /\  ph ) )
 
Theoremmoani 2306 "At most one" is still true when a conjunct is added. (Contributed by NM, 9-Mar-1995.)
 |- 
 E* x ph   =>    |- 
 E* x ( ps 
 /\  ph )
 
Theoremmoor 2307 "At most one" is still the case when a disjunct is removed. (Contributed by NM, 5-Apr-2004.)
 |-  ( E* x (
 ph  \/  ps )  ->  E* x ph )
 
Theoremmooran1 2308 "At most one" imports disjunction to conjunction. (Contributed by NM, 5-Apr-2004.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |-  ( ( E* x ph 
 \/  E* x ps )  ->  E* x ( ph  /\ 
 ps ) )
 
Theoremmooran2 2309 "At most one" exports disjunction to conjunction. (Contributed by NM, 5-Apr-2004.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |-  ( E* x (
 ph  \/  ps )  ->  ( E* x ph  /\ 
 E* x ps )
 )
 
Theoremmoanim 2310 Introduction of a conjunct into "at most one" quantifier. (Contributed by NM, 3-Dec-2001.)
 |- 
 F/ x ph   =>    |-  ( E* x (
 ph  /\  ps )  <->  (
 ph  ->  E* x ps )
 )
 
Theoremeuan 2311 Introduction of a conjunct into uniqueness quantifier. (Contributed by NM, 19-Feb-2005.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |- 
 F/ x ph   =>    |-  ( E! x (
 ph  /\  ps )  <->  (
 ph  /\  E! x ps ) )
 
Theoremmoanimv 2312* Introduction of a conjunct into "at most one" quantifier. (Contributed by NM, 23-Mar-1995.)
 |-  ( E* x (
 ph  /\  ps )  <->  (
 ph  ->  E* x ps )
 )
 
Theoremmoaneu 2313 Nested "at most one" and uniqueness quantifiers. (Contributed by NM, 25-Jan-2006.)
 |- 
 E* x ( ph  /\ 
 E! x ph )
 
Theoremmoanmo 2314 Nested "at most one" quantifiers. (Contributed by NM, 25-Jan-2006.)
 |- 
 E* x ( ph  /\ 
 E* x ph )
 
Theoremeuanv 2315* Introduction of a conjunct into uniqueness quantifier. (Contributed by NM, 23-Mar-1995.)
 |-  ( E! x (
 ph  /\  ps )  <->  (
 ph  /\  E! x ps ) )
 
Theoremmopick 2316 "At most one" picks a variable value, eliminating an existential quantifier. (Contributed by NM, 27-Jan-1997.)
 |-  ( ( E* x ph 
 /\  E. x ( ph  /\ 
 ps ) )  ->  ( ph  ->  ps )
 )
 
Theoremeupick 2317 Existential uniqueness "picks" a variable value for which another wff is true. If there is only one thing  x such that 
ph is true, and there is also an  x (actually the same one) such that  ph and  ps are both true, then  ph implies  ps regardless of  x. This theorem can be useful for eliminating existential quantifiers in a hypothesis. Compare Theorem *14.26 in [WhiteheadRussell] p. 192. (Contributed by NM, 10-Jul-1994.)
 |-  ( ( E! x ph 
 /\  E. x ( ph  /\ 
 ps ) )  ->  ( ph  ->  ps )
 )
 
Theoremeupicka 2318 Version of eupick 2317 with closed formulas. (Contributed by NM, 6-Sep-2008.)
 |-  ( ( E! x ph 
 /\  E. x ( ph  /\ 
 ps ) )  ->  A. x ( ph  ->  ps ) )
 
Theoremeupickb 2319 Existential uniqueness "pick" showing wff equivalence. (Contributed by NM, 25-Nov-1994.)
 |-  ( ( E! x ph 
 /\  E! x ps  /\  E. x ( ph  /\  ps ) )  ->  ( ph  <->  ps ) )
 
Theoremeupickbi 2320 Theorem *14.26 in [WhiteheadRussell] p. 192. (Contributed by Andrew Salmon, 11-Jul-2011.)
 |-  ( E! x ph  ->  ( E. x (
 ph  /\  ps )  <->  A. x ( ph  ->  ps ) ) )
 
Theoremmopick2 2321 "At most one" can show the existence of a common value. In this case we can infer existence of conjunction from a conjunction of existence, and it is one way to achieve the converse of 19.40 1616. (Contributed by NM, 5-Apr-2004.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |-  ( ( E* x ph 
 /\  E. x ( ph  /\ 
 ps )  /\  E. x ( ph  /\  ch ) )  ->  E. x ( ph  /\  ps  /\  ch ) )
 
Theoremeuor2 2322 Introduce or eliminate a disjunct in a uniqueness quantifier. (Contributed by NM, 21-Oct-2005.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |-  ( -.  E. x ph 
 ->  ( E! x (
 ph  \/  ps )  <->  E! x ps ) )
 
Theoremmoexex 2323 "At most one" double quantification. (Contributed by NM, 3-Dec-2001.)
 |- 
 F/ y ph   =>    |-  ( ( E* x ph 
 /\  A. x E* y ps )  ->  E* y E. x ( ph  /\  ps ) )
 
Theoremmoexexv 2324* "At most one" double quantification. (Contributed by NM, 26-Jan-1997.)
 |-  ( ( E* x ph 
 /\  A. x E* y ps )  ->  E* y E. x ( ph  /\  ps ) )
 
Theorem2moex 2325 Double quantification with "at most one." (Contributed by NM, 3-Dec-2001.)
 |-  ( E* x E. y ph  ->  A. y E* x ph )
 
Theorem2euex 2326 Double quantification with existential uniqueness. (Contributed by NM, 3-Dec-2001.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |-  ( E! x E. y ph  ->  E. y E! x ph )
 
Theorem2eumo 2327 Double quantification with existential uniqueness and "at most one." (Contributed by NM, 3-Dec-2001.)
 |-  ( E! x E* y ph  ->  E* x E! y ph )
 
Theorem2eu2ex 2328 Double existential uniqueness. (Contributed by NM, 3-Dec-2001.)
 |-  ( E! x E! y ph  ->  E. x E. y ph )
 
Theorem2moswap 2329 A condition allowing swap of "at most one" and existential quantifiers. (Contributed by NM, 10-Apr-2004.)
 |-  ( A. x E* y ph  ->  ( E* x E. y ph  ->  E* y E. x ph ) )
 
Theorem2euswap 2330 A condition allowing swap of uniqueness and existential quantifiers. (Contributed by NM, 10-Apr-2004.)
 |-  ( A. x E* y ph  ->  ( E! x E. y ph  ->  E! y E. x ph ) )
 
Theorem2exeu 2331 Double existential uniqueness implies double uniqueness quantification. (Contributed by NM, 3-Dec-2001.) (Proof shortened by Mario Carneiro, 22-Dec-2016.)
 |-  ( ( E! x E. y ph  /\  E! y E. x ph )  ->  E! x E! y ph )
 
Theorem2mo 2332* Two equivalent expressions for double "at most one." (Contributed by NM, 2-Feb-2005.) (Revised by Mario Carneiro, 17-Oct-2016.)
 |-  ( E. z E. w A. x A. y
 ( ph  ->  ( x  =  z  /\  y  =  w ) )  <->  A. x A. y A. z A. w ( ( ph  /\  [
 z  /  x ] [ w  /  y ] ph )  ->  ( x  =  z  /\  y  =  w )
 ) )
 
Theorem2mos 2333* Double "exists at most one", using implicit substitution. (Contributed by NM, 10-Feb-2005.)
 |-  ( ( x  =  z  /\  y  =  w )  ->  ( ph 
 <->  ps ) )   =>    |-  ( E. z E. w A. x A. y ( ph  ->  ( x  =  z  /\  y  =  w )
 ) 
 <-> 
 A. x A. y A. z A. w ( ( ph  /\  ps )  ->  ( x  =  z  /\  y  =  w ) ) )
 
Theorem2eu1 2334 Double existential uniqueness. This theorem shows a condition under which a "naive" definition matches the correct one. (Contributed by NM, 3-Dec-2001.)
 |-  ( A. x E* y ph  ->  ( E! x E! y ph  <->  ( E! x E. y ph  /\  E! y E. x ph )
 ) )
 
Theorem2eu2 2335 Double existential uniqueness. (Contributed by NM, 3-Dec-2001.)
 |-  ( E! y E. x ph  ->  ( E! x E! y ph  <->  E! x E. y ph ) )
 
Theorem2eu3 2336 Double existential uniqueness. (Contributed by NM, 3-Dec-2001.)
 |-  ( A. x A. y ( E* x ph 
 \/  E* y ph )  ->  ( ( E! x E! y ph  /\  E! y E! x ph )  <->  ( E! x E. y ph  /\  E! y E. x ph ) ) )
 
Theorem2eu4 2337* This theorem provides us with a definition of double existential uniqueness ("exactly one 
x and exactly one  y"). Naively one might think (incorrectly) that it could be defined by  E! x E! y ph. See 2eu1 2334 for a condition under which the naive definition holds and 2exeu 2331 for a one-way implication. See 2eu5 2338 and 2eu8 2341 for alternate definitions. (Contributed by NM, 3-Dec-2001.)
 |-  ( ( E! x E. y ph  /\  E! y E. x ph )  <->  ( E. x E. y ph  /\  E. z E. w A. x A. y
 ( ph  ->  ( x  =  z  /\  y  =  w ) ) ) )
 
Theorem2eu5 2338* An alternate definition of double existential uniqueness (see 2eu4 2337). A mistake sometimes made in the literature is to use  E! x E! y to mean "exactly one  x and exactly one  y." (For example, see Proposition 7.53 of [TakeutiZaring] p. 53.) It turns out that this is actually a weaker assertion, as can be seen by expanding out the formal definitions. This theorem shows that the erroneous definition can be repaired by conjoining 
A. x E* y ph as an additional condition. The correct definition apparently has never been published. ( E* means "exists at most one.") (Contributed by NM, 26-Oct-2003.)
 |-  ( ( E! x E! y ph  /\  A. x E* y ph )  <->  ( E. x E. y ph  /\  E. z E. w A. x A. y
 ( ph  ->  ( x  =  z  /\  y  =  w ) ) ) )
 
Theorem2eu6 2339* Two equivalent expressions for double existential uniqueness. (Contributed by NM, 2-Feb-2005.) (Revised by Mario Carneiro, 17-Oct-2016.)
 |-  ( ( E! x E. y ph  /\  E! y E. x ph )  <->  E. z E. w A. x A. y ( ph  <->  ( x  =  z  /\  y  =  w )
 ) )
 
Theorem2eu7 2340 Two equivalent expressions for double existential uniqueness. (Contributed by NM, 19-Feb-2005.)
 |-  ( ( E! x E. y ph  /\  E! y E. x ph )  <->  E! x E! y ( E. x ph  /\  E. y ph ) )
 
Theorem2eu8 2341 Two equivalent expressions for double existential uniqueness. Curiously, we can put  E! on either of the internal conjuncts but not both. We can also commute  E! x E! y using 2eu7 2340. (Contributed by NM, 20-Feb-2005.)
 |-  ( E! x E! y ( E. x ph 
 /\  E. y ph )  <->  E! x E! y ( E! x ph  /\  E. y ph ) )
 
Theoremeuequ1 2342* Equality has existential uniqueness. Special case of eueq1 3067 proved using only predicate calculus. (Contributed by Stefan Allan, 4-Dec-2008.)
 |- 
 E! x  x  =  y
 
Theoremexists1 2343* Two ways to express "only one thing exists." The left-hand side requires only one variable to express this. Both sides are false in set theory; see theorem dtru 4350. (Contributed by NM, 5-Apr-2004.)
 |-  ( E! x  x  =  x  <->  A. x  x  =  y )
 
Theoremexists2 2344 A condition implying that at least two things exist. (Contributed by NM, 10-Apr-2004.) (Proof shortened by Andrew Salmon, 9-Jul-2011.)
 |-  ( ( E. x ph 
 /\  E. x  -.  ph )  ->  -.  E! x  x  =  x )
 
1.8  Other axiomatizations related to classical predicate calculus
 
1.8.1  Predicate calculus with all distinct variables
 
Axiomax-7d 2345* Distinct variable version of ax-7 1745. (Contributed by Mario Carneiro, 14-Aug-2015.)
 |-  ( A. x A. y ph  ->  A. y A. x ph )
 
Axiomax-8d 2346* Distinct variable version of ax-8 1683. (Contributed by Mario Carneiro, 14-Aug-2015.)
 |-  ( x  =  y 
 ->  ( x  =  z 
 ->  y  =  z
 ) )
 
Axiomax-9d1 2347 Distinct variable version of ax-9 1662, equal variables case. (Contributed by Mario Carneiro, 14-Aug-2015.)
 |- 
 -.  A. x  -.  x  =  x
 
Axiomax-9d2 2348* Distinct variable version of ax-9 1662, distinct variables case. (Contributed by Mario Carneiro, 14-Aug-2015.)
 |- 
 -.  A. x  -.  x  =  y
 
Axiomax-10d 2349* Distinct variable version of ax10 1991. (Contributed by Mario Carneiro, 14-Aug-2015.)
 |-  ( A. x  x  =  y  ->  A. y  y  =  x )
 
Axiomax-11d 2350* Distinct variable version of ax-11 1757. (Contributed by Mario Carneiro, 14-Aug-2015.)
 |-  ( x  =  y 
 ->  ( A. y ph  ->  A. x ( x  =  y  ->  ph )
 ) )
 
1.8.2  Aristotelian logic: Assertic syllogisms

Model the Aristotelian assertic syllogisms using modern notation. This section shows that the Aristotelian assertic syllogisms can be proven with our axioms of logic, and also provides generally useful theorems.

In antiquity Aristotelian logic and Stoic logic (see mpto1 1539) were the leading logical systems. Aristotelian logic became the leading system in medieval Europe; this section models this system (including later refinements to it). Aristotle defined syllogisms very generally ("a discourse in which certain (specific) things having been supposed, something different from the things supposed results of necessity because these things are so") Aristotle, Prior Analytics 24b18-20. However, in Prior Analytics he limits himself to categorical syllogisms that consist of three categorical propositions with specific structures. The syllogisms are the valid subset of the possible combinations of these structures. The medieval schools used vowels to identify the types of terms (a=all, e=none, i=some, and o=some are not), and named the different syllogisms with Latin words that had the vowels in the intended order.

"There is a surprising amount of scholarly debate about how best to formalize Aristotle's syllogisms..." according to Aristotle's Modal Proofs: Prior Analytics A8-22 in Predicate Logic, Adriane Rini, Springer, 2011, ISBN 978-94-007-0049-9, page 28. For example, Lukasiewicz believes it is important to note that "Aristotle does not introduce singular terms or premisses into his system". Lukasiewicz also believes that Aristotelian syllogisms are predicates (having a true/false value), not inference rules: "The characteristic sign of an inference is the word 'therefore'... no syllogism is formulated by Aristotle primarily as an inference, but they are all implications." Jan Lukasiewicz, Aristotle's Syllogistic from the Standpoint of Modern Formal Logic, Second edition, Oxford, 1957, page 1-2. Lukasiewicz devised a specialized prefix notation for representing Aristotelian syllogisms instead of using standard predicate logic notation.

We instead translate each Aristotelian syllogism into an inference rule, and each rule is defined using standard predicate logic notation and predicates. The predicates are represented by wff variables that may depend on the quantified variable  x. Our translation is essentially identical to the one use in Rini page 18, Table 2 "Non-Modal Syllogisms in Lower Predicate Calculus (LPC)", which uses standard predicate logic with predicates. Rini states, "the crucial point is that we capture the meaning Aristotle intends, and the method by which we represent that meaning is less important." There are two differences: we make the existence criteria explicit, and we use  ph,  ps, and  ch in the order they appear (a common Metamath convention). Patzig also uses standard predicate logic notation and predicates (though he interprets them as conditional propositions, not as inference rules); see Gunther Patzig, Aristotle's Theory of the Syllogism second edition, 1963, English translation by Jonathan Barnes, 1968, page 38. Terms such as "all" and "some" are translated into predicate logic using the aproach devised by Frege and Russell. "Frege (and Russell) devised an ingenious procedure for regimenting binary quantifiers like "every" and "some" in terms of unary quantifiers like "everything" and "something": they formalized sentences of the form "Some A is B" and "Every A is B" as exists x (Ax and Bx) and all x (Ax implies Bx), respectively." "Quantifiers and Quantification", Stanford Encyclopedia of Philosophy, http://plato.stanford.edu/entries/quantification/. See Principia Mathematica page 22 and *10 for more information (especially *10.3 and *10.26).

Expressions of the form "no  ph is  ps " are consistently translated as  A. x (
ph  ->  -.  ps ). These can also be expressed as  -.  E. x
( ph  /\  ps ), per alinexa 1585. We translate "all  ph is  ps " to  A. x (
ph  ->  ps ), "some  ph is  ps " to  E. x
( ph  /\  ps ), and "some  ph is not  ps " to  E. x
( ph  /\  -.  ps ). It is traditional to use the singular verb "is", not the plural verb "are", in the generic expressions. By convention the major premise is listed first.

In traditional Aristotelian syllogisms the predicates have a restricted form ("x is a ..."); those predicates could be modeled in modern notation by constructs such as  x  =  A,  x  e.  A, or  x  C_  A. Here we use wff variables instead of specialized restricted forms. This generalization makes the syllogisms more useful in more circumstances. In addition, these expressions make it clearer that the syllogisms of Aristolean logic are the forerunners of predicate calculus. If we used restricted forms like  x  e.  A instead, we would not only unnecessarily limit their use, but we would also need to use set and class axioms, making their relationship to predicate calculus less clear.

There are some widespread misconceptions about the existential assumptions made by Aristotle (aka "existential import"). Aristotle was not trying to develop something exactly corresponding to modern logic. Aristotle devised "a companion-logic for science. He relegates fictions like fairy godmothers and mermaids and unicorns to the realms of poetry and literature. In his mind, they exist outside the ambit of science. This is why he leaves no room for such non-existent entities in his logic. This is a thoughtful choice, not an inadvertent omission. Technically, Aristotelian science is a search for definitions, where a definition is "a phrase signifying a thing's essence." (Topics, I.5.102a37, Pickard-Cambridge.)... Because non-existent entities cannot be anything, they do not, in Aristotle's mind, possess an essence... This is why he leaves no place for fictional entities like goat-stags (or unicorns)." Source: Louis F. Groarke, "Aristotle: Logic", section 7. (Existential Assumptions), Internet Encyclopedia of Philosophy (A Peer-Reviewed Academic Resource), http://www.iep.utm.edu/aris-log/. Thus, some syllogisms have "extra" existence hypotheses that do not directly appear in Aristotle's original materials (since they were always assumed); they are added where they are needed. This affects barbari 2355, celaront 2356, cesaro 2361, camestros 2362, felapton 2367, darapti 2368, calemos 2372, fesapo 2373, and bamalip 2374.

These are only the assertic syllogisms. Aristotle also defined modal syllogisms that deal with modal qualifiers such as "necessarily" and "possibly". Historically Aristotelian modal syllogisms were not as widely used. For more about modal syllogisms in a modern context, see Rini as well as Aristotle's Modal Syllogistic by Marko Malink, Harvard University Press, November 2013. We do not treat them further here.

Aristotelean logic is essentially the forerunner of predicate calculus (as well as set theory since it discusses membership in groups), while Stoic logic is essentially the forerunner of propositional calculus.

 
Theorembarbara 2351 "Barbara", one of the fundamental syllogisms of Aristotelian logic. All  ph is  ps, and all  ch is  ph, therefore all  ch is  ps. (In Aristotelian notation, AAA-1: MaP and SaM therefore SaP.) For example, given "All men are mortal" and "Socrates is a man", we can prove "Socrates is mortal". If H is the set of men, M is the set of mortal beings, and S is Socrates, these word phrases can be represented as  A. x ( x  e.  H  ->  x  e.  M ) (all men are mortal) and  A. x ( x  =  S  ->  x  e.  H ) (Socrates is a man) therefore  A. x ( x  =  S  ->  x  e.  M ) (Socrates is mortal). Russell and Whitehead note that the "syllogism in Barbara is derived..." from syl 16. (quote after Theorem *2.06 of [WhiteheadRussell] p. 101). Most of the proof is in alsyl 1622. There are a legion of sources for Barbara, including http://www.friesian.com/aristotl.htm, http://plato.stanford.edu/entries/aristotle-logic/, and https://en.wikipedia.org/wiki/Syllogism. (Contributed by David A. Wheeler, 24-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ch  ->  ph )   =>    |-  A. x ( ch  ->  ps )
 
Theoremcelarent 2352 "Celarent", one of the syllogisms of Aristotelian logic. No  ph is  ps, and all  ch is  ph, therefore no  ch is  ps. (In Aristotelian notation, EAE-1: MeP and SaM therefore SeP.) For example, given the "No reptiles have fur" and "All snakes are reptiles", therefore "No snakes have fur". Example from https://en.wikipedia.org/wiki/Syllogism. (Contributed by David A. Wheeler, 24-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  A. x ( ch  ->  ph )   =>    |-  A. x ( ch  ->  -.  ps )
 
Theoremdarii 2353 "Darii", one of the syllogisms of Aristotelian logic. All  ph is  ps, and some  ch is  ph, therefore some  ch is  ps. (In Aristotelian notation, AII-1: MaP and SiM therefore SiP.) For example, given "All rabbits have fur" and "Some pets are rabbits", therefore "Some pets have fur". Example from https://en.wikipedia.org/wiki/Syllogism. (Contributed by David A. Wheeler, 24-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  E. x ( ch  /\  ph )   =>    |-  E. x ( ch  /\  ps )
 
Theoremferio 2354 "Ferio" ("Ferioque"), one of the syllogisms of Aristotelian logic. No  ph is  ps, and some  ch is  ph, therefore some  ch is not  ps. (In Aristotelian notation, EIO-1: MeP and SiM therefore SoP.) For example, given "No homework is fun" and "Some reading is homework", therefore "Some reading is not fun". This is essentially a logical axiom in Aristotelian logic. Example from https://en.wikipedia.org/wiki/Syllogism. (Contributed by David A. Wheeler, 24-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  E. x ( ch  /\  ph )   =>    |-  E. x ( ch  /\  -.  ps )
 
Theorembarbari 2355 "Barbari", one of the syllogisms of Aristotelian logic. All  ph is  ps, all  ch is  ph, and some  ch exist, therefore some  ch is  ps. (In Aristotelian notation, AAI-1: MaP and SaM therefore SiP.) For example, given "All men are mortal", "All Greeks are men", and "Greeks exist", therefore "Some Greeks are mortal". Note the existence hypothesis (to prove the "some" in the conclusion). Example from https://en.wikipedia.org/wiki/Syllogism. (Contributed by David A. Wheeler, 27-Aug-2016.) (Revised by David A. Wheeler, 30-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ch  ->  ph )   &    |-  E. x ch   =>    |- 
 E. x ( ch 
 /\  ps )
 
Theoremcelaront 2356 "Celaront", one of the syllogisms of Aristotelian logic. No  ph is  ps, all  ch is  ph, and some  ch exist, therefore some  ch is not  ps. (In Aristotelian notation, EAO-1: MeP and SaM therefore SoP.) For example, given "No reptiles have fur", "All snakes are reptiles.", and "Snakes exist.", prove "Some snakes have no fur". Note the existence hypothesis. Example from https://en.wikipedia.org/wiki/Syllogism. (Contributed by David A. Wheeler, 27-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  A. x ( ch  ->  ph )   &    |-  E. x ch   =>    |- 
 E. x ( ch 
 /\  -.  ps )
 
Theoremcesare 2357 "Cesare", one of the syllogisms of Aristotelian logic. No  ph is  ps, and all  ch is  ps, therefore no  ch is  ph. (In Aristotelian notation, EAE-2: PeM and SaM therefore SeP.) Related to celarent 2352. (Contributed by David A. Wheeler, 27-Aug-2016.) (Revised by David A. Wheeler, 13-Nov-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  A. x ( ch  ->  ps )   =>    |-  A. x ( ch  ->  -.  ph )
 
Theoremcamestres 2358 "Camestres", one of the syllogisms of Aristotelian logic. All  ph is  ps, and no  ch is  ps, therefore no  ch is  ph. (In Aristotelian notation, AEE-2: PaM and SeM therefore SeP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ch  ->  -.  ps )   =>    |-  A. x ( ch  ->  -.  ph )
 
Theoremfestino 2359 "Festino", one of the syllogisms of Aristotelian logic. No  ph is  ps, and some  ch is  ps, therefore some  ch is not  ph. (In Aristotelian notation, EIO-2: PeM and SiM therefore SoP.) (Contributed by David A. Wheeler, 25-Nov-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  E. x ( ch  /\  ps )   =>    |-  E. x ( ch  /\  -.  ph )
 
Theorembaroco 2360 "Baroco", one of the syllogisms of Aristotelian logic. All  ph is  ps, and some  ch is not  ps, therefore some  ch is not  ph. (In Aristotelian notation, AOO-2: PaM and SoM therefore SoP.) For example, "All informative things are useful", "Some websites are not useful", therefore "Some websites are not informative." (Contributed by David A. Wheeler, 28-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  E. x ( ch  /\  -.  ps )   =>    |- 
 E. x ( ch 
 /\  -.  ph )
 
Theoremcesaro 2361 "Cesaro", one of the syllogisms of Aristotelian logic. No  ph is  ps, all  ch is  ps, and  ch exist, therefore some  ch is not  ph. (In Aristotelian notation, EAO-2: PeM and SaM therefore SoP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  A. x ( ch  ->  ps )   &    |-  E. x ch   =>    |- 
 E. x ( ch 
 /\  -.  ph )
 
Theoremcamestros 2362 "Camestros", one of the syllogisms of Aristotelian logic. All  ph is  ps, no  ch is  ps, and  ch exist, therefore some  ch is not  ph. (In Aristotelian notation, AEO-2: PaM and SeM therefore SoP.) For example, "All horses have hooves", "No humans have hooves", and humans exist, therefore "Some humans are not horses". (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ch  ->  -.  ps )   &    |-  E. x ch   =>    |- 
 E. x ( ch 
 /\  -.  ph )
 
Theoremdatisi 2363 "Datisi", one of the syllogisms of Aristotelian logic. All  ph is  ps, and some  ph is  ch, therefore some  ch is  ps. (In Aristotelian notation, AII-3: MaP and MiS therefore SiP.) (Contributed by David A. Wheeler, 28-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  E. x (
 ph  /\  ch )   =>    |-  E. x ( ch  /\  ps )
 
Theoremdisamis 2364 "Disamis", one of the syllogisms of Aristotelian logic. Some  ph is  ps, and all  ph is  ch, therefore some  ch is  ps. (In Aristotelian notation, IAI-3: MiP and MaS therefore SiP.) (Contributed by David A. Wheeler, 28-Aug-2016.)
 |- 
 E. x ( ph  /\ 
 ps )   &    |-  A. x (
 ph  ->  ch )   =>    |- 
 E. x ( ch 
 /\  ps )
 
Theoremferison 2365 "Ferison", one of the syllogisms of Aristotelian logic. No  ph is  ps, and some  ph is  ch, therefore some  ch is not  ps. (In Aristotelian notation, EIO-3: MeP and MiS therefore SoP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  E. x ( ph  /\  ch )   =>    |-  E. x ( ch  /\  -.  ps )
 
Theorembocardo 2366 "Bocardo", one of the syllogisms of Aristotelian logic. Some  ph is not  ps, and all  ph is  ch, therefore some  ch is not  ps. (In Aristotelian notation, OAO-3: MoP and MaS therefore SoP.) For example, "Some cats have no tails", "All cats are mammals", therefore "Some mammals have no tails". A reorder of disamis 2364; prefer using that instead. (Contributed by David A. Wheeler, 28-Aug-2016.) (New usage is discouraged.)
 |- 
 E. x ( ph  /\ 
 -.  ps )   &    |-  A. x (
 ph  ->  ch )   =>    |- 
 E. x ( ch 
 /\  -.  ps )
 
Theoremfelapton 2367 "Felapton", one of the syllogisms of Aristotelian logic. No  ph is  ps, all  ph is  ch, and some  ph exist, therefore some  ch is not  ps. (In Aristotelian notation, EAO-3: MeP and MaS therefore SoP.) For example, "No flowers are animals" and "All flowers are plants", therefore "Some plants are not animals". (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  A. x ( ph  ->  ch )   &    |-  E. x ph   =>    |-  E. x ( ch  /\  -. 
 ps )
 
Theoremdarapti 2368 "Darapti", one of the syllogisms of Aristotelian logic. All  ph is  ps, all  ph is  ch, and some  ph exist, therefore some  ch is  ps. (In Aristotelian notation, AAI-3: MaP and MaS therefore SiP.) For example, "All squares are rectangles" and "All squares are rhombuses", therefore "Some rhombuses are rectangles". (Contributed by David A. Wheeler, 28-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x (
 ph  ->  ch )   &    |-  E. x ph   =>    |-  E. x ( ch  /\  ps )
 
Theoremcalemes 2369 "Calemes", one of the syllogisms of Aristotelian logic. All  ph is  ps, and no  ps is  ch, therefore no  ch is  ph. (In Aristotelian notation, AEE-4: PaM and MeS therefore SeP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ps  ->  -.  ch )   =>    |-  A. x ( ch  ->  -.  ph )
 
Theoremdimatis 2370 "Dimatis", one of the syllogisms of Aristotelian logic. Some  ph is  ps, and all  ps is  ch, therefore some  ch is  ph. (In Aristotelian notation, IAI-4: PiM and MaS therefore SiP.) For example, "Some pets are rabbits.", "All rabbits have fur", therefore "Some fur bearing animals are pets". Like darii 2353 with positions interchanged. (Contributed by David A. Wheeler, 28-Aug-2016.)
 |- 
 E. x ( ph  /\ 
 ps )   &    |-  A. x ( ps  ->  ch )   =>    |-  E. x ( ch  /\  ph )
 
Theoremfresison 2371 "Fresison", one of the syllogisms of Aristotelian logic. No  ph is  ps (PeM), and some  ps is  ch (MiS), therefore some  ch is not  ph (SoP). (In Aristotelian notation, EIO-4: PeM and MiS therefore SoP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  E. x ( ps  /\  ch )   =>    |-  E. x ( ch  /\  -.  ph )
 
Theoremcalemos 2372 "Calemos", one of the syllogisms of Aristotelian logic. All  ph is  ps (PaM), no  ps is  ch (MeS), and  ch exist, therefore some  ch is not  ph (SoP). (In Aristotelian notation, AEO-4: PaM and MeS therefore SoP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ps  ->  -.  ch )   &    |-  E. x ch   =>    |- 
 E. x ( ch 
 /\  -.  ph )
 
Theoremfesapo 2373 "Fesapo", one of the syllogisms of Aristotelian logic. No  ph is  ps, all  ps is  ch, and  ps exist, therefore some  ch is not  ph. (In Aristotelian notation, EAO-4: PeM and MaS therefore SoP.) (Contributed by David A. Wheeler, 28-Aug-2016.) (Revised by David A. Wheeler, 2-Sep-2016.)
 |- 
 A. x ( ph  ->  -.  ps )   &    |-  A. x ( ps  ->  ch )   &    |-  E. x ps   =>    |- 
 E. x ( ch 
 /\  -.  ph )
 
Theorembamalip 2374 "Bamalip", one of the syllogisms of Aristotelian logic. All  ph is  ps, all  ps is  ch, and  ph exist, therefore some  ch is  ph. (In Aristotelian notation, AAI-4: PaM and MaS therefore SiP.) Like barbari 2355. (Contributed by David A. Wheeler, 28-Aug-2016.)
 |- 
 A. x ( ph  ->  ps )   &    |-  A. x ( ps  ->  ch )   &    |-  E. x ph   =>    |-  E. x ( ch  /\  ph )
 
1.8.3  Intuitionistic logic

Intuitionistic (constructive) logic is similar to classical logic with the notable omission of ax-3 7 and theorems such as exmid 405 or peirce 174. We mostly treat intuitionistic logic in a separate file, iset.mm, which is known as the Intuitionistic Logic Explorer on the web site. However, iset.mm has a number of additional axioms (mainly to replace definitions like df-or 360 and df-ex 1548 which are not valid in intitionistic logic) and we want to prove those axioms here to demonstrate that adding those axioms in iset.mm does not make iset.mm any less consistent than set.mm.

 
Theoremaxi4 2375 Specialization (intuitionistic logic axiom ax-4). This is just sp 1759 by another name. (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( A. x ph  -> 
 ph )
 
Theoremaxi5r 2376 Converse of ax-5o (intuitionistic logic axiom ax-i5r). (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( ( A. x ph 
 ->  A. x ps )  ->  A. x ( A. x ph  ->  ps )
 )
 
Theoremaxial 2377  x is not free in  A. x ph (intuitionistic logic axiom ax-ial). (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( A. x ph  ->  A. x A. x ph )
 
Theoremaxie1 2378  x is bound in  E. x ph (intuitionistic logic axiom ax-ie1). (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( E. x ph  ->  A. x E. x ph )
 
Theoremaxie2 2379 A key property of existential quantification (intuitionistic logic axiom ax-ie2). (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( A. x ( ps  ->  A. x ps )  ->  ( A. x ( ph  ->  ps )  <->  ( E. x ph  ->  ps ) ) )
 
Theoremaxi9 2380 Axiom of existence (intuitionistic logic axiom ax-i9). In classical logic, this is equivalent to ax-9 1662 but in intuitionistic logic it needs to be stated using the existential quantifier. (Contributed by Jim Kingdon, 31-Dec-2017.)
 |- 
 E. x  x  =  y
 
Theoremaxi10 2381 Axiom of Quantifier Substitution (intuitionistic logic axiom ax-10). This is just ax10 1991 by another name. (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( A. x  x  =  y  ->  A. y  y  =  x )
 
Theoremaxi11e 2382 Axiom of Variable Substitution for Existence (intuitionistic logic axiom ax-i11e). This can be derived from ax-11 1757 in a classical context but a separate axiom is needed for intuitionistic predicate calculus. (Contributed by Jim Kingdon, 31-Dec-2017.)
 |-  ( x  =  y 
 ->  ( E. x ( x  =  y  /\  ph )  ->  E. y ph ) )
 
Theoremaxi12 2383 Axiom of Quantifier Introduction (intuitionistic logic axiom ax-i12).

In classical logic, this is mostly a restatement of ax12o 1976 (with one additional quantifier). But in intuitionistic logic, changing the negations and implications to disjunctions makes it stronger.

(Contributed by Jim Kingdon, 31-Dec-2017.)

 |-  ( A. z  z  =  x  \/  ( A. z  z  =  y  \/  A. z ( x  =  y  ->  A. z  x  =  y ) ) )
 
Theoremaxbnd 2384 Axiom of Bundling (intuitionistic logic axiom ax-bnd).

In classical logic, this and axi12 2383 are fairly straightforward consequences of ax12o 1976. But in intuitionistic logic, it is not easy to add the extra  A. x to axi12 2383 and so we treat the two as separate axioms.

(Contributed by Jim Kingdon, 22-Mar-2018.)

 |-  ( A. z  z  =  x  \/  ( A. z  z  =  y  \/  A. x A. z ( x  =  y  ->  A. z  x  =  y ) ) )
 
PART 2  ZF (ZERMELO-FRAENKEL) SET THEORY

Set theory uses the formalism of propositional and predicate calculus to assert properties of arbitrary mathematical objects called "sets." A set can be contained in another set, and this relationship is indicated by the  e. symbol. Starting with the simplest mathematical object, called the empty set, set theory builds up more and more complex structures whose existence follows from the axioms, eventually resulting in extremely complicated sets that we identify with the real numbers and other familiar mathematical objects.

A simplistic concept of sets, sometimes called "naive set theory", is vulnerable to a paradox called "Russell's paradox" (ru 3120), a discovery that revolutionized the foundations of mathematics and logic. Russell's Paradox spawned the development of set theories that countered the paradox, including the ZF set theory that is most widely used and is defined here.

Except for Extensionality, the axioms basically say, "given an arbitrary set x (and, in the cases of Replacement and Regularity, provided that an antecedent is satisfied), there exists another set y based on or constructed from it, with the stated properties." (The axiom of Extensionality can also be restated this way as shown by axext2 2386.) The individual axiom links provide more detailed descriptions. We derive the redundant ZF axioms of Separation, Null Set, and Pairing from the others as theorems.

 
2.1  ZF Set Theory - start with the Axiom of Extensionality
 
2.1.1  Introduce the Axiom of Extensionality
 
Axiomax-ext 2385* Axiom of Extensionality. An axiom of Zermelo-Fraenkel set theory. It states that two sets are identical if they contain the same elements. Axiom Ext of [BellMachover] p. 461.

Set theory can also be formulated with a single primitive predicate  e. on top of traditional predicate calculus without equality. In that case the Axiom of Extensionality becomes  ( A. w
( w  e.  x  <->  w  e.  y )  -> 
( x  e.  z  ->  y  e.  z ) ), and equality  x  =  y is defined as  A. w ( w  e.  x  <->  w  e.  y
). All of the usual axioms of equality then become theorems of set theory. See, for example, Axiom 1 of [TakeutiZaring] p. 8.

To use the above "equality-free" version of Extensionality with Metamath's logical axioms, we would rewrite ax-8 1683 through ax-16 2194 with equality expanded according to the above definition. Some of those axioms could be proved from set theory and would be redundant. Not all of them are redundant, since our axioms of predicate calculus make essential use of equality for the proper substitution that is a primitive notion in traditional predicate calculus. A study of such an axiomatization would be an interesting project for someone exploring the foundations of logic.

General remarks: Our set theory axioms are presented using defined connectives (
<->, 
E., etc.) for convenience. However, it is implicitly understood that the actual axioms use only the primitive connectives  ->,  -.,  A.,  =, and  e.. It is straightforward to establish the equivalence between the actual axioms and the ones we display, and we will not do so.

It is important to understand that strictly speaking, all of our set theory axioms are really schemes that represent an infinite number of actual axioms. This is inherent in the design of Metamath ("metavariable math"), which manipulates only metavariables. For example, the metavariable  x in ax-ext 2385 can represent any actual variable v1, v2, v3,... . Distinct variable restrictions ($d) prevent us from substituting say v1 for both  x and  z. This is in contrast to typical textbook presentations that present actual axioms (except for Replacement ax-rep 4280, which involves a wff metavariable). In practice, though, the theorems and proofs are essentially the same. The $d restrictions make each of the infinite axioms generated by the ax-ext 2385 scheme exactly logically equivalent to each other and in particular to the actual axiom of the textbook version. (Contributed by NM, 5-Aug-1993.)

 |-  ( A. z ( z  e.  x  <->  z  e.  y
 )  ->  x  =  y )
 
Theoremaxext2 2386* The Axiom of Extensionality (ax-ext 2385) restated so that it postulates the existence of a set  z given two arbitrary sets 
x and  y. This way to express it follows the general idea of the other ZFC axioms, which is to postulate the existence of sets given other sets. (Contributed by NM, 28-Sep-2003.)
 |- 
 E. z ( ( z  e.  x  <->  z  e.  y
 )  ->  x  =  y )
 
Theoremaxext3 2387* A generalization of the Axiom of Extensionality in which  x and  y need not be distinct. (Contributed by NM, 15-Sep-1993.) (Proof shortened by Andrew Salmon, 12-Aug-2011.)
 |-  ( A. z ( z  e.  x  <->  z  e.  y
 )  ->  x  =  y )
 
Theoremaxext4 2388* A bidirectional version of Extensionality. Although this theorem "looks" like it is just a definition of equality, it requires the Axiom of Extensionality for its proof under our axiomatization. See the comments for ax-ext 2385 and df-cleq 2397. (Contributed by NM, 14-Nov-2008.)
 |-  ( x  =  y  <->  A. z ( z  e.  x  <->  z  e.  y
 ) )
 
Theorembm1.1 2389* Any set defined by a property is the only set defined by that property. Theorem 1.1 of [BellMachover] p. 462. (Contributed by NM, 30-Jun-1994.)
 |- 
 F/ x ph   =>    |-  ( E. x A. y ( y  e.  x  <->  ph )  ->  E! x A. y ( y  e.  x  <->  ph ) )
 
2.1.2  Class abstractions (a.k.a. class builders)
 
Syntaxcab 2390 Introduce the class builder or class abstraction notation ("the class of sets  x such that  ph is true"). Our class variables  A,  B, etc. range over class builders (implicitly in the case of defined class terms such as df-nul 3589). Note that a set variable can be expressed as a class builder per theorem cvjust 2399, justifying the assignment of set variables to class variables via the use of cv 1648.
 class  { x  |  ph }
 
Definitiondf-clab 2391 Define class abstraction notation (so-called by Quine), also called a "class builder" in the literature.  x and  y need not be distinct. Definition 2.1 of [Quine] p. 16. Typically,  ph will have  y as a free variable, and " { y  |  ph } " is read "the class of all sets  y such that  ph ( y ) is true." We do not define  { y  |  ph } in isolation but only as part of an expression that extends or "overloads" the  e. relationship.

This is our first use of the 
e. symbol to connect classes instead of sets. The syntax definition wcel 1721, which extends or "overloads" the wel 1722 definition connecting set variables, requires that both sides of  e. be a class. In df-cleq 2397 and df-clel 2400, we introduce a new kind of variable (class variable) that can substituted with expressions such as  { y  | 
ph }. In the present definition, the  x on the left-hand side is a set variable. Syntax definition cv 1648 allows us to substitute a set variable  x for a class variable: all sets are classes by cvjust 2399 (but not necessarily vice-versa). For a full description of how classes are introduced and how to recover the primitive language, see the discussion in Quine (and under abeq2 2509 for a quick overview).

Because class variables can be substituted with compound expressions and set variables cannot, it is often useful to convert a theorem containing a free set variable to a more general version with a class variable. This is done with theorems such as vtoclg 2971 which is used, for example, to convert elirrv 7521 to elirr 7522.

This is called the "axiom of class comprehension" by [Levy] p. 338, who treats the theory of classes as an extralogical extension to our logic and set theory axioms. He calls the construction  {
y  |  ph } a "class term".

For a general discussion of the theory of classes, see http://us.metamath.org/mpeuni/mmset.html#class. (Contributed by NM, 5-Aug-1993.)

 |-  ( x  e.  {
 y  |  ph }  <->  [ x  /  y ] ph )
 
Theoremabid 2392 Simplification of class abstraction notation when the free and bound variables are identical. (Contributed by NM, 5-Aug-1993.)
 |-  ( x  e.  { x  |  ph }  <->  ph )
 
Theoremhbab1 2393* Bound-variable hypothesis builder for a class abstraction. (Contributed by NM, 5-Aug-1993.)
 |-  ( y  e.  { x  |  ph }  ->  A. x  y  e.  { x  |  ph } )
 
Theoremnfsab1 2394* Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 11-Aug-2016.)
 |- 
 F/ x  y  e. 
 { x  |  ph }
 
Theoremhbab 2395* Bound-variable hypothesis builder for a class abstraction. (Contributed by NM, 1-Mar-1995.)
 |-  ( ph  ->  A. x ph )   =>    |-  ( z  e.  {
 y  |  ph }  ->  A. x  z  e.  {
 y  |  ph } )
 
Theoremnfsab 2396* Bound-variable hypothesis builder for a class abstraction. (Contributed by Mario Carneiro, 11-Aug-2016.)
 |- 
 F/ x ph   =>    |- 
 F/ x  z  e. 
 { y  |  ph }
 
Definitiondf-cleq 2397* Define the equality connective between classes. Definition 2.7 of [Quine] p. 18. Also Definition 4.5 of [TakeutiZaring] p. 13; Chapter 4 provides its justification and methods for eliminating it. Note that its elimination will not necessarily result in a single wff in the original language but possibly a "scheme" of wffs.

This is an example of a somewhat "risky" definition, meaning that it has a more complex than usual soundness justification (outside of Metamath), because it "overloads" or reuses the existing equality symbol rather than introducing a new symbol. This allows us to make statements that may not hold for the original symbol. For example, it permits us to deduce  y  =  z  <->  A. x ( x  e.  y  <->  x  e.  z
), which is not a theorem of logic but rather presupposes the Axiom of Extensionality (see theorem axext4 2388). We therefore include this axiom as a hypothesis, so that the use of Extensionality is properly indicated.

We could avoid this complication by introducing a new symbol, say =2, in place of  =. This would also have the advantage of making elimination of the definition straightforward, so that we could eliminate Extensionality as a hypothesis. We would then also have the advantage of being able to identify in various proofs exactly where Extensionality truly comes into play rather than just being an artifact of a definition. One of our theorems would then be  x =2  y  <->  x  =  y by invoking Extensionality.

However, to conform to literature usage, we retain this overloaded definition. This also makes some proofs shorter and probably easier to read, without the constant switching between two kinds of equality.

See also comments under df-clab 2391, df-clel 2400, and abeq2 2509.

In the form of dfcleq 2398, this is called the "axiom of extensionality" by [Levy] p. 338, who treats the theory of classes as an extralogical extension to our logic and set theory axioms.

For a general discussion of the theory of classes, see http://us.metamath.org/mpeuni/mmset.html#class. (Contributed by NM, 15-Sep-1993.)

 |-  ( A. x ( x  e.  y  <->  x  e.  z
 )  ->  y  =  z )   =>    |-  ( A  =  B  <->  A. x ( x  e.  A  <->  x  e.  B ) )
 
Theoremdfcleq 2398* The same as df-cleq 2397 with the hypothesis removed using the Axiom of Extensionality ax-ext 2385. (Contributed by NM, 15-Sep-1993.)
 |-  ( A  =  B  <->  A. x ( x  e.  A  <->  x  e.  B ) )
 
Theoremcvjust 2399* Every set is a class. Proposition 4.9 of [TakeutiZaring] p. 13. This theorem shows that a set variable can be expressed as a class abstraction. This provides a motivation for the class syntax construction cv 1648, which allows us to substitute a set variable for a class variable. See also cab 2390 and df-clab 2391. Note that this is not a rigorous justification, because cv 1648 is used as part of the proof of this theorem, but a careful argument can be made outside of the formalism of Metamath, for example as is done in Chapter 4 of Takeuti and Zaring. See also the discussion under the definition of class in [Jech] p. 4 showing that "Every set can be considered to be a class." (Contributed by NM, 7-Nov-2006.)
 |-  x  =  { y  |  y  e.  x }
 
Definitiondf-clel 2400* Define the membership connective between classes. Theorem 6.3 of [Quine] p. 41, or Proposition 4.6 of [TakeutiZaring] p. 13, which we adopt as a definition. See these references for its metalogical justification. Note that like df-cleq 2397 it extends or "overloads" the use of the existing membership symbol, but unlike df-cleq 2397 it does not strengthen the set of valid wffs of logic when the class variables are replaced with set variables (see cleljust 2064), so we don't include any set theory axiom as a hypothesis. See also comments about the syntax under df-clab 2391. Alternate definitions of  A  e.  B (but that require either  A or  B to be a set) are shown by clel2 3032, clel3 3034, and clel4 3035.

This is called the "axiom of membership" by [Levy] p. 338, who treats the theory of classes as an extralogical extension to our logic and set theory axioms.

For a general discussion of the theory of classes, see http://us.metamath.org/mpeuni/mmset.html#class. (Contributed by NM, 5-Aug-1993.)

 |-  ( A  e.  B  <->  E. x ( x  =  A  /\  x  e.  B ) )
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