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Morse–Kelley set theory

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The axioms and definitions in this section are, but for a few inessential details, taken from the Appendix to Kelley (1955). The explanatory remarks below are not his. The Appendix states 181 theorems and definitions, and warrants careful reading as an abbreviated exposition of axiomatic set theory
2297:(representing second-order objects in set rather than predicate language) as its background logic. The language of second-order ZFC is similar to that of MK (although a set and a class having the same extension can no longer be identified), and their 2146:) may range over all sets.) The NBG axiom schema of Class Comprehension can be replaced with finitely many of its instances; this is not possible in MK. MK is consistent relative to ZFC augmented by an axiom asserting the existence of strongly 1474: 512: 140:
of ZFC. Unlike von Neumann–Bernays–Gödel set theory, where the axiom schema of Class Comprehension can be replaced with finitely many of its instances, Morse–Kelley set theory cannot be finitely axiomatized.
910: 1820: 1640: 2837:' by the formula obtained from Æ by replacing each occurrence of the variable that replaced α by the variable that replaced β provided that the variable that replaced β does not appear bound in 346: 2138:) may contain quantified variables ranging over classes. The quantified variables in NBG's axiom schema of Class Comprehension are restricted to sets; hence Class Comprehension in NBG must be 1116: 406: 2781: 2305:
of second-order ZFC are quite different from those of MK. For example, if MK is consistent then it has a countable first-order model, while second-order ZFC has no countable models.
3346: 723: 3668: 1208: 1155: 3303: 280: 3764:. However, the summary of ML given in Mendelson (1997), p. 296, is easier to follow. Mendelson's axiom schema ML2 is identical to the above axiom schema of Class Comprehension. 3406: 3444: 2039: 3510: 2554: 3529: 2901: 1884: 305: 136:(ZFC, the canonical set theory) in the sense that a statement in the language of ZFC is provable in NBG if and only if it is provable in ZFC, Morse–Kelley set theory is a 1678: 1254: 636: 3615: 3226: 965: 2249: 2222: 759: 431: 3473: 3118: 3029: 2930: 2691: 2269: 2801: 2743: 2662: 2614: 3070: 235: 191:, appearing in Extensionality, Class Comprehension, and Foundation, denote variables ranging over classes. A lower case letter denotes a variable that cannot be a 2824: 4777: 2473:
by a working mathematician of the first rank. Kelley introduced his axioms gradually, as needed to develop the topics listed after each instance of
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A set and a class having the same extension are identical. Hence MK is not a two-sorted theory, appearances to the contrary notwithstanding.
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resources for practical proof are almost identical (and are identical if MK includes the strong form of Limitation of Size). But the
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in Power Set and Union are universally, not existentially, quantified, as Class Comprehension suffices to establish the existence of
3535:. Up to this point, everything that has been proved to exist is a class, and Kelley's discussion of sets was entirely hypothetical. 2130:. In fact, NBG—and hence ZFC—can be proved consistent in MK. MK's strength stems from its axiom schema of Class Comprehension being 5849: 2194:. Such an axiom could be added, of course, and minor perturbations of the above axioms would necessitate this addition. The set 2111:. These authors and Mendelson (1997: 287) submit that MK does what is expected of a set theory while being less cumbersome than 2282:. It cannot be a set (under pain of paradox); hence that class is a proper class, and all proper classes have the same size as 771: 6007: 1702: 1522: 4795: 5862: 5185: 4203: 4016: 79: 2426:
is equivalent to Kelley's, but formulated in an idiosyncratic formal language rather than, as is done here, in standard
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is also a set. Replacement can prove everything that Limitation of Size proves, except prove some form of the
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Notations appearing below and now well-known are not defined. Peculiarities of Kelley's notation include:
681: 183:, inessential details aside. The symbolic versions of the axioms employ the following notational devices: 6159: 6154: 5764: 5503: 4761: 4662: 4157: 4120: 3638: 3184: 2180: 2154: 1480: 1222: 1160: 1128: 3975:(1955) by John L. Kelley in various formats. The appendix contains Kelley's axiomatic development of MK. 3282: 244: 6088: 5678: 5072: 5040: 4731: 3385: 20: 4174: 3705:
If the scope of all quantified variables in the above axioms is restricted to sets, all axioms except
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to range over sets alone, Morse–Kelley set theory allows these bound variables to range over
2786: 2728: 2641: 2593: 6244: 6206: 6083: 5887: 5727: 5651: 5629: 5457: 5415: 5314: 5281: 5145: 4933: 4844: 4570: 4560: 4394: 4325: 4278: 4218: 4098: 3984: 3938: 3155: 3147: 3049: 2506: 2166: 1487:. The next section explains how Limitation of Size is stronger than the usual forms of the 1236: 412: 211: 63: 3970: 179:
With the exception of Class Comprehension, the following axioms are the same as those for
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Monk (1980) and Rubin (1967) are set theory texts built around MK; Rubin's
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distinguish variables ranging over classes from those ranging over sets;
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on sets as sets of ordered pairs, making possible the next axiom:
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This axiom, or equivalents thereto, are included in ZFC and NBG.
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The above axioms are shared with other set theories as follows:
70:(NBG). While von Neumann–Bernays–Gödel set theory restricts the 4426: 4248: 2342: 2298: 1505: 5508: 4854: 4699: 4298: 4058: 3994: 3862:"Some impredicative definitions in the axiomatic set theory" 3835:
The definitive treatment of the closely related set theory
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ZFC, NBG, and MK each have models describable in terms of
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Pairing licenses the unordered pair in terms of which the
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and Morse. Morse's own version appeared later in his book
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From Foundations of Mathematics (FOM) discussion group:
905:{\displaystyle \forall W_{1}...W_{n}\exists Y\forall x.} 2165:, and an "axiom of replacement," asserting that if the 1815:{\displaystyle \forall a\,\forall s\,)\rightarrow Ms].} 1635:{\displaystyle \forall a\,\forall p\,)\rightarrow Mp].} 164:. Classes that are members of other classes are called 3839:, followed by a page on MK. Harder than Monk or Rubin. 3076:
is a set because it is a subclass of the power set of
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Morse–Kelley set theory is named after mathematicians
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asserts the unconditional existence of two sets, the
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would follow from either form of Limitation of Size.
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Classes having the same members are the same class.
2293:MK can be confused with second-order ZFC, ZFC with 2060:. Power Set and Union only serve to establish that 358:having all possible sets as members, is defined by 3815: 3717:is provable in ZFC. Hence the Kelley treatment of 3662: 3609: 3523: 3504: 3467: 3438: 3400: 3340: 3297: 3220: 3112: 3064: 3023: 2924: 2895: 2818: 2795: 2775: 2737: 2685: 2656: 2608: 2548: 2263: 2243: 2216: 2033: 1878: 1814: 1672: 1634: 1468: 1202: 1149: 1110: 959: 904: 753: 717: 630: 506: 400: 341:{\displaystyle \forall x(x\not \in \varnothing ).} 340: 299: 274: 229: 6414: 3729:as well as sets, and the Classification schema. 648:) be any formula in the language of MK in which 128:While von Neumann–Bernays–Gödel set theory is a 3846:. Krieger. Easier and less thorough than Rubin. 1479:The formal version of this axiom resembles the 109:and later in an appendix to Kelley's textbook 4670: 4010: 3744:See, e.g., Mendelson (1997), p. 239, axiom R. 3721:makes very clear that all that distinguishes 2942:above. Sketch of the proof of Power Set from 3694:of the axiom of choice. As is the case with 3654: 3648: 3427: 3421: 3107: 3095: 3059: 3053: 2770: 2758: 2540: 2519: 2498:denote the domain and range of the function 1870: 1864: 1426: 1414: 1402: 1390: 1343: 1331: 1197: 1194: 1182: 1176: 1170: 1167: 1144: 1132: 954: 942: 712: 691: 3042:above. Sketch of the proof of Pairing from 4862: 4677: 4663: 4017: 4003: 3531:is a set simply because it is a member of 2126:NBG, the other well-known set theory with 1504:be a class whose members are all possible 74:in the schematic formula appearing in the 3946: 3920: 3880: 3856: 2829:'α' and 'β' are replaced by variables, ' 2715:includes proper classes as well as sets. 2453:class comprehension was also proposed in 2122:MK is strictly stronger than ZFC and its 1763: 1741: 1719: 1712: 1583: 1561: 1539: 1532: 1111:{\displaystyle \forall x\,\forall y\,)].} 1083: 1079: 1051: 1032: 998: 991: 458: 448: 441: 144: 113:(1955), a graduate level introduction to 2505:His primitive logical language includes 401:{\displaystyle \forall x(Mx\to x\in V).} 3985:Allen Hazen on set theory with classes. 2776:{\displaystyle \beta \in \{\alpha :A\}} 2377:) is a model of Mendelson's version of 86:as well as sets, as first suggested by 6415: 4684: 2082:: Pairing, Power Set, Union, Infinity; 1157:, may be defined in the usual way, as 4658: 3998: 3341:{\displaystyle x\cap y=\varnothing .} 670:this is the only way MK differs from 237:whose intended reading is "the class 3990:Joseph Shoenfield's doubts about MK. 3903: 3800:Introduction to Axiomatic Set Theory 3725:from ZFC are variables ranging over 2411: 718:{\displaystyle Y=\{x\mid \phi (x)\}} 106: 68:von Neumann–Bernays–Gödel set theory 3663:{\displaystyle V-\{\varnothing \}.} 2430:. The first set theory to include 1203:{\displaystyle \ \{\{x\},\{x,y\}\}} 1150:{\displaystyle \langle x,y\rangle } 13: 3821:Introduction to Mathematical Logic 3298:{\displaystyle x\neq \varnothing } 2414:and popularized in an appendix to 2169:of a class function is a set, its 1968: 1947: 1926: 1896: 1757: 1735: 1713: 1706: 1577: 1555: 1533: 1526: 1483:, and embodies the class function 1378: 1372: 1366: 1310: 1292: 1283: 1271: 1262: 1045: 1026: 992: 985: 813: 807: 775: 586: 565: 544: 534:from at least one of its members. 452: 442: 435: 365: 314: 275:{\displaystyle \exists W(x\in W).} 248: 187:The upper case letters other than 14: 6434: 3964: 3651: 3401:{\displaystyle \varnothing \in y} 3332: 3292: 1914: 559: 329: 168:. A class that is not a set is a 16:System of mathematical set theory 6396: 4048: 3894:Set Theory for the Mathematician 3439:{\displaystyle x\cup \{x\}\in y} 3247:may be combined into one axiom. 725:whose members are exactly those 176:involve membership or equality. 3631:There exists a choice function 2308: 2251:In this case, the existence of 1365: 4024: 3767: 3747: 3738: 3702:requires some form of choice. 3598: 3592: 2953:that is a subclass of the set 2537: 2531: 2034:{\displaystyle \exists y)])].} 2025: 2022: 2019: 2016: 2013: 1989: 1986: 1974: 1953: 1944: 1932: 1902: 1831:There exists an inductive set 1806: 1797: 1794: 1791: 1788: 1764: 1754: 1742: 1723: 1720: 1626: 1617: 1614: 1611: 1608: 1596: 1584: 1574: 1562: 1543: 1540: 1456: 1453: 1450: 1438: 1435: 1387: 1384: 1355: 1352: 1316: 1307: 1298: 1289: 1280: 1268: 1102: 1099: 1096: 1093: 1067: 1064: 1052: 1033: 1023: 1020: 1002: 999: 896: 893: 881: 840: 834: 831: 819: 748: 742: 709: 703: 622: 619: 616: 604: 592: 571: 562: 550: 498: 486: 483: 471: 459: 449: 392: 380: 371: 332: 320: 266: 254: 1: 6357:History of mathematical logic 3909:Proc. Natl. Acad. Sci. U.S.A. 3802:. Routledge & Kegan Paul. 3779: 3505:{\displaystyle \varnothing .} 2869:is a set, there exists a set 2549:{\displaystyle \ \{x:A(x)\},} 2098: 761:comes out true. Formally, if 6282:Primitive recursive function 3773:Kelley (1955), p. 261, fn †. 3524:{\displaystyle \varnothing } 2896:{\displaystyle z\subseteq x} 2719:II. Classification (schema): 2319:von Neumann universe of sets 1879:{\displaystyle x\cup \{x\}.} 1680:be the sum class of the set 300:{\displaystyle \varnothing } 7: 3185:axiom schema of replacement 2983:is not a set. Existence of 2855:and of the universal class 2711:, except that the scope of 2228:could be a set larger than 2198:is not identified with the 2155:axiom of limitation of size 1673:{\displaystyle s=\bigcup a} 1481:axiom schema of replacement 134:Zermelo–Fraenkel set theory 66:that is closely related to 10: 6439: 5346:Schröder–Bernstein theorem 5073:Monadic predicate calculus 4732:Foundations of mathematics 4515:von Neumann–Bernays–Gödel 3844:Introduction to Set Theory 3812:. Oxford: Basil Blackwell. 3757:for ML is the 1951 ed. of 2703:would be identical to the 2405: 2153:The only advantage of the 2068:cannot be proper classes. 967:whose members are exactly 631:{\displaystyle \forall A.} 21:foundations of mathematics 6392: 6379:Philosophy of mathematics 6328:Automated theorem proving 6310: 6205: 6037: 5930: 5782: 5499: 5475: 5453:Von Neumann–Bernays–Gödel 5398: 5292: 5196: 5094: 5085: 5012: 4947: 4853: 4775: 4692: 4579: 4542: 4454: 4344: 4316:One-to-one correspondence 4232: 4173: 4057: 4046: 4032: 3698:, the development of the 3610:{\displaystyle c(x)\in x} 3221:{\displaystyle \bigcup x} 2290:too can be well-ordered. 960:{\displaystyle z=\{x,y\}} 105:and was first set out by 57:system of Quine and Morse 3732: 3080:(by two applications of 2833:' by a formula Æ, and ' 2634:if and only if for each 2434:class comprehension was 2410:MK was first set out in 2244:{\displaystyle \omega .} 2217:{\displaystyle \omega ,} 754:{\displaystyle \phi (x)} 6029:Self-verifying theories 5850:Tarski's axiomatization 4801:Tarski's undefinability 4796:incompleteness theorems 3882:10.4064/fm-37-1-111-124 3869:Fundamenta Mathematicae 3842:Monk, J. Donald (1980) 3676:is very similar to the 3468:{\displaystyle x\in y.} 3113:{\displaystyle \{x,y\}} 3024:{\displaystyle x\cup y} 2961:is a member of the set 2925:{\displaystyle z\in y.} 2721:An axiom results if in 2705:axiom of extensionality 2686:{\displaystyle z\in y.} 2556:"the class of all sets 2465:The axioms in Kelley's 2264:{\displaystyle \omega } 2157:is that it implies the 1835:, meaning that (i) the 241:is a set", abbreviates 90:in 1940 for his system 41:Morse–Tarski set theory 33:Kelley–Morse set theory 25:Morse–Kelley set theory 6403:Mathematics portal 6014:Proof of impossibility 5662:propositional variable 4972:Propositional calculus 4274:Constructible universe 4094:Constructibility (V=L) 3823:. Chapman & Hall. 3678:axiom of global choice 3664: 3611: 3525: 3506: 3469: 3440: 3402: 3342: 3299: 3222: 3114: 3066: 3025: 2926: 2897: 2820: 2797: 2796:{\displaystyle \beta } 2777: 2739: 2738:{\displaystyle \beta } 2687: 2658: 2657:{\displaystyle z\in x} 2610: 2609:{\displaystyle x\in y} 2550: 2351:constructible universe 2265: 2245: 2218: 2159:axiom of global choice 2148:inaccessible cardinals 2124:conservative extension 2035: 1880: 1816: 1674: 1636: 1470: 1204: 1151: 1112: 961: 906: 755: 719: 675:. Then there exists a 632: 508: 402: 342: 301: 276: 231: 152:and MK share a common 145:MK axioms and ontology 130:conservative extension 49:Quine–Morse set theory 6423:Systems of set theory 6272:Kolmogorov complexity 6225:Computably enumerable 6125:Model complete theory 5917:Principia Mathematica 4977:Propositional formula 4806:Banach–Tarski paradox 4497:Principia Mathematica 4331:Transfinite induction 4190:(i.e. set difference) 3922:10.1073/pnas.35.3.150 3849:Morse, A. P., (1965) 3665: 3612: 3526: 3507: 3470: 3441: 3403: 3370:transfinite induction 3343: 3300: 3223: 3115: 3067: 3065:{\displaystyle \{x\}} 3026: 2927: 2898: 2821: 2798: 2778: 2740: 2688: 2659: 2611: 2551: 2327:inaccessible cardinal 2266: 2246: 2219: 2094:: Limitation of Size. 2036: 1881: 1817: 1675: 1637: 1471: 1205: 1152: 1113: 962: 929:, there exists a set 907: 756: 720: 633: 509: 403: 343: 302: 277: 232: 230:{\displaystyle \ Mx,} 158:universe of discourse 6220:Church–Turing thesis 6207:Computability theory 5416:continuum hypothesis 4934:Square of opposition 4792:Gödel's completeness 4571:Burali-Forti paradox 4326:Set-builder notation 4279:Continuum hypothesis 4219:Symmetric difference 3639: 3586: 3515: 3493: 3450: 3412: 3386: 3317: 3283: 3209: 3156:function composition 3092: 3050: 3009: 3005:are both sets, then 2907: 2881: 2807: 2787: 2749: 2729: 2668: 2642: 2594: 2513: 2276:von Neumann ordinals 2255: 2232: 2205: 1893: 1855: 1703: 1655: 1523: 1255: 1161: 1129: 982: 933: 772: 736: 682: 642:Class Comprehension: 541: 526:Each nonempty class 432: 413:von Neumann universe 362: 311: 291: 245: 212: 64:axiomatic set theory 6374:Mathematical object 6265:P versus NP problem 6230:Computable function 6024:Reverse mathematics 5950:Logical consequence 5827:primitive recursive 5822:elementary function 5595:Free/bound variable 5448:Tarski–Grothendieck 4967:Logical connectives 4897:Logical equivalence 4747:Logical consequence 4532:Tarski–Grothendieck 3489:, and the null set 3378:There exists a set 3168:is a function and 2873:such that for each 2851:. Existence of the 2664:when and only when 2401:, is a model of MK. 80:Class Comprehension 6172:Transfer principle 6135:Semantics of logic 6120:Categorical theory 6096:Non-standard model 5610:Logical connective 4737:Information theory 4686:Mathematical logic 4121:Limitation of size 3858:Mostowski, Andrzej 3817:Mendelson, Elliott 3762:Mathematical Logic 3682:Limitation of Size 3660: 3607: 3582:is a function and 3521: 3502: 3465: 3436: 3398: 3338: 3305:there is a member 3295: 3218: 3110: 3062: 3021: 2922: 2893: 2819:{\displaystyle B,} 2816: 2793: 2773: 2735: 2683: 2654: 2606: 2546: 2295:second-order logic 2261: 2241: 2214: 2192:axiom of empty set 2181:Limitation of Size 2031: 1876: 1812: 1688:of all members of 1670: 1632: 1466: 1464: 1223:Limitation of Size 1200: 1147: 1108: 957: 902: 765:is not free in φ: 751: 715: 628: 504: 398: 338: 297: 272: 227: 6410: 6409: 6342:Abstract category 6145:Theories of truth 5955:Rule of inference 5945:Natural deduction 5926: 5925: 5471: 5470: 5176:Cartesian product 5081: 5080: 4987:Many-valued logic 4962:Boolean functions 4845:Russell's paradox 4820:diagonal argument 4717:First-order logic 4652: 4651: 4561:Russell's paradox 4510:Zermelo–Fraenkel 4411:Dedekind-infinite 4284:Diagonal argument 4183:Cartesian product 4040:Set (mathematics) 3853:. Academic Press. 3255:Cartesian product 3199:VI. Amalgamation: 2582:(and hence not a 2518: 2428:first-order logic 2329:κ be a member of 2134:, meaning that φ( 1242:mapped one-to-one 1166: 217: 6430: 6401: 6400: 6352:History of logic 6347:Category of sets 6240:Decision problem 6019:Ordinal analysis 5960:Sequent calculus 5858:Boolean algebras 5798: 5797: 5772: 5743:logical/constant 5497: 5496: 5483: 5406:Zermelo–Fraenkel 5157:Set operations: 5092: 5091: 5029: 4860: 4859: 4840:Löwenheim–Skolem 4727:Formal semantics 4679: 4672: 4665: 4656: 4655: 4634:Bertrand Russell 4624:John von Neumann 4609:Abraham Fraenkel 4604:Richard Dedekind 4566:Suslin's problem 4477:Cantor's theorem 4194:De Morgan's laws 4052: 4019: 4012: 4005: 3996: 3995: 3973:General Topology 3959: 3950: 3924: 3885: 3884: 3866: 3851:A Theory of Sets 3834: 3810:Parts of Classes 3790:General Topology 3774: 3771: 3765: 3751: 3745: 3742: 3713:are ZFC axioms. 3700:cardinal numbers 3669: 3667: 3666: 3661: 3635:whose domain is 3617:for each member 3616: 3614: 3613: 3608: 3559:rational numbers 3530: 3528: 3527: 3522: 3511: 3509: 3508: 3503: 3474: 3472: 3471: 3466: 3445: 3443: 3442: 3437: 3407: 3405: 3404: 3399: 3347: 3345: 3344: 3339: 3304: 3302: 3301: 3296: 3277:VII. Regularity: 3227: 3225: 3224: 3219: 3162:V. Substitution: 3134:: Unordered and 3119: 3117: 3116: 3111: 3071: 3069: 3068: 3063: 3046:: the singleton 3030: 3028: 3027: 3022: 2965:whose existence 2931: 2929: 2928: 2923: 2902: 2900: 2899: 2894: 2825: 2823: 2822: 2817: 2802: 2800: 2799: 2794: 2782: 2780: 2779: 2774: 2744: 2742: 2741: 2736: 2692: 2690: 2689: 2684: 2663: 2661: 2660: 2655: 2615: 2613: 2612: 2607: 2555: 2553: 2552: 2547: 2516: 2467:General Topology 2441:, that built on 2424:A Theory of Sets 2420:General Topology 2270: 2268: 2267: 2262: 2250: 2248: 2247: 2242: 2223: 2221: 2220: 2215: 2040: 2038: 2037: 2032: 1885: 1883: 1882: 1877: 1821: 1819: 1818: 1813: 1679: 1677: 1676: 1671: 1641: 1639: 1638: 1633: 1475: 1473: 1472: 1467: 1465: 1209: 1207: 1206: 1201: 1164: 1156: 1154: 1153: 1148: 1117: 1115: 1114: 1109: 966: 964: 963: 958: 911: 909: 908: 903: 880: 879: 858: 857: 806: 805: 787: 786: 760: 758: 757: 752: 724: 722: 721: 716: 637: 635: 634: 629: 513: 511: 510: 505: 407: 405: 404: 399: 347: 345: 344: 339: 306: 304: 303: 298: 281: 279: 278: 273: 236: 234: 233: 228: 215: 174:atomic sentences 172:. The primitive 138:proper extension 123:A Theory of Sets 111:General Topology 6438: 6437: 6433: 6432: 6431: 6429: 6428: 6427: 6413: 6412: 6411: 6406: 6395: 6388: 6333:Category theory 6323:Algebraic logic 6306: 6277:Lambda calculus 6215:Church encoding 6201: 6177:Truth predicate 6033: 5999:Complete theory 5922: 5791: 5787: 5783: 5778: 5770: 5490: and  5486: 5481: 5467: 5443:New Foundations 5411:axiom of choice 5394: 5356:Gödel numbering 5296: and  5288: 5192: 5077: 5027: 5008: 4957:Boolean algebra 4943: 4907:Equiconsistency 4872:Classical logic 4849: 4830:Halting problem 4818: and  4794: and  4782: and  4781: 4776:Theorems ( 4771: 4688: 4683: 4653: 4648: 4575: 4554: 4538: 4503:New Foundations 4450: 4340: 4259:Cardinal number 4242: 4228: 4169: 4053: 4044: 4028: 4023: 3967: 3864: 3831: 3782: 3777: 3772: 3768: 3752: 3748: 3743: 3739: 3735: 3709:and the schema 3680:derivable from 3640: 3637: 3636: 3587: 3584: 3583: 3576:choice function 3543:Natural numbers 3516: 3513: 3512: 3494: 3491: 3490: 3451: 3448: 3447: 3413: 3410: 3409: 3387: 3384: 3383: 3376:VIII. Infinity: 3366:Ordinal numbers 3318: 3315: 3314: 3284: 3281: 3280: 3210: 3207: 3206: 3205:is a set, then 3183:is that of the 3172:is a set, then 3093: 3090: 3089: 3051: 3048: 3047: 3010: 3007: 3006: 2969:asserts. Hence 2908: 2905: 2904: 2882: 2879: 2878: 2849:algebra of sets 2808: 2805: 2804: 2788: 2785: 2784: 2783:if and only if 2750: 2747: 2746: 2730: 2727: 2726: 2669: 2666: 2665: 2643: 2640: 2639: 2595: 2592: 2591: 2586:) if, for some 2514: 2511: 2510: 2507:class abstracts 2470: 2445:rather than on 2443:New Foundations 2408: 2400: 2389: 2376: 2362: 2340: 2333:. Also let Def( 2311: 2256: 2253: 2252: 2233: 2230: 2229: 2206: 2203: 2202: 2175:axiom of choice 2163:axiom of choice 2101: 1894: 1891: 1890: 1856: 1853: 1852: 1847:is a member of 1839:is a member of 1704: 1701: 1700: 1656: 1653: 1652: 1524: 1521: 1520: 1489:axiom of choice 1463: 1462: 1362: 1361: 1258: 1256: 1253: 1252: 1234:if and only if 1162: 1159: 1158: 1130: 1127: 1126: 983: 980: 979: 934: 931: 930: 875: 871: 853: 849: 801: 797: 782: 778: 773: 770: 769: 737: 734: 733: 683: 680: 679: 660:is not free. φ( 542: 539: 538: 433: 430: 429: 363: 360: 359: 356:universal class 312: 309: 308: 292: 289: 288: 246: 243: 242: 213: 210: 209: 147: 72:bound variables 17: 12: 11: 5: 6436: 6426: 6425: 6408: 6407: 6393: 6390: 6389: 6387: 6386: 6381: 6376: 6371: 6366: 6365: 6364: 6354: 6349: 6344: 6335: 6330: 6325: 6320: 6318:Abstract logic 6314: 6312: 6308: 6307: 6305: 6304: 6299: 6297:Turing machine 6294: 6289: 6284: 6279: 6274: 6269: 6268: 6267: 6262: 6257: 6252: 6247: 6237: 6235:Computable set 6232: 6227: 6222: 6217: 6211: 6209: 6203: 6202: 6200: 6199: 6194: 6189: 6184: 6179: 6174: 6169: 6164: 6163: 6162: 6157: 6152: 6142: 6137: 6132: 6130:Satisfiability 6127: 6122: 6117: 6116: 6115: 6105: 6104: 6103: 6093: 6092: 6091: 6086: 6081: 6076: 6071: 6061: 6060: 6059: 6054: 6047:Interpretation 6043: 6041: 6035: 6034: 6032: 6031: 6026: 6021: 6016: 6011: 6001: 5996: 5995: 5994: 5993: 5992: 5982: 5977: 5967: 5962: 5957: 5952: 5947: 5942: 5936: 5934: 5928: 5927: 5924: 5923: 5921: 5920: 5912: 5911: 5910: 5909: 5904: 5903: 5902: 5897: 5892: 5872: 5871: 5870: 5868:minimal axioms 5865: 5854: 5853: 5852: 5841: 5840: 5839: 5834: 5829: 5824: 5819: 5814: 5801: 5799: 5780: 5779: 5777: 5776: 5775: 5774: 5762: 5757: 5756: 5755: 5750: 5745: 5740: 5730: 5725: 5720: 5715: 5714: 5713: 5708: 5698: 5697: 5696: 5691: 5686: 5681: 5671: 5666: 5665: 5664: 5659: 5654: 5644: 5643: 5642: 5637: 5632: 5627: 5622: 5617: 5607: 5602: 5597: 5592: 5591: 5590: 5585: 5580: 5575: 5565: 5560: 5558:Formation rule 5555: 5550: 5549: 5548: 5543: 5533: 5532: 5531: 5521: 5516: 5511: 5506: 5500: 5494: 5477:Formal systems 5473: 5472: 5469: 5468: 5466: 5465: 5460: 5455: 5450: 5445: 5440: 5435: 5430: 5425: 5420: 5419: 5418: 5413: 5402: 5400: 5396: 5395: 5393: 5392: 5391: 5390: 5380: 5375: 5374: 5373: 5366:Large cardinal 5363: 5358: 5353: 5348: 5343: 5329: 5328: 5327: 5322: 5317: 5302: 5300: 5290: 5289: 5287: 5286: 5285: 5284: 5279: 5274: 5264: 5259: 5254: 5249: 5244: 5239: 5234: 5229: 5224: 5219: 5214: 5209: 5203: 5201: 5194: 5193: 5191: 5190: 5189: 5188: 5183: 5178: 5173: 5168: 5163: 5155: 5154: 5153: 5148: 5138: 5133: 5131:Extensionality 5128: 5126:Ordinal number 5123: 5113: 5108: 5107: 5106: 5095: 5089: 5083: 5082: 5079: 5078: 5076: 5075: 5070: 5065: 5060: 5055: 5050: 5045: 5044: 5043: 5033: 5032: 5031: 5018: 5016: 5010: 5009: 5007: 5006: 5005: 5004: 4999: 4994: 4984: 4979: 4974: 4969: 4964: 4959: 4953: 4951: 4945: 4944: 4942: 4941: 4936: 4931: 4926: 4921: 4916: 4911: 4910: 4909: 4899: 4894: 4889: 4884: 4879: 4874: 4868: 4866: 4857: 4851: 4850: 4848: 4847: 4842: 4837: 4832: 4827: 4822: 4810:Cantor's  4808: 4803: 4798: 4788: 4786: 4773: 4772: 4770: 4769: 4764: 4759: 4754: 4749: 4744: 4739: 4734: 4729: 4724: 4719: 4714: 4709: 4708: 4707: 4696: 4694: 4690: 4689: 4682: 4681: 4674: 4667: 4659: 4650: 4649: 4647: 4646: 4641: 4639:Thoralf Skolem 4636: 4631: 4626: 4621: 4616: 4611: 4606: 4601: 4596: 4591: 4585: 4583: 4577: 4576: 4574: 4573: 4568: 4563: 4557: 4555: 4553: 4552: 4549: 4543: 4540: 4539: 4537: 4536: 4535: 4534: 4529: 4524: 4523: 4522: 4507: 4506: 4505: 4493: 4492: 4491: 4480: 4479: 4474: 4469: 4464: 4458: 4456: 4452: 4451: 4449: 4448: 4443: 4438: 4433: 4424: 4419: 4414: 4404: 4399: 4398: 4397: 4392: 4387: 4377: 4367: 4362: 4357: 4351: 4349: 4342: 4341: 4339: 4338: 4333: 4328: 4323: 4321:Ordinal number 4318: 4313: 4308: 4303: 4302: 4301: 4296: 4286: 4281: 4276: 4271: 4266: 4256: 4251: 4245: 4243: 4241: 4240: 4237: 4233: 4230: 4229: 4227: 4226: 4221: 4216: 4211: 4206: 4201: 4199:Disjoint union 4196: 4191: 4185: 4179: 4177: 4171: 4170: 4168: 4167: 4166: 4165: 4160: 4149: 4148: 4146:Martin's axiom 4143: 4138: 4133: 4128: 4123: 4118: 4113: 4111:Extensionality 4108: 4107: 4106: 4096: 4091: 4090: 4089: 4084: 4079: 4069: 4063: 4061: 4055: 4054: 4047: 4045: 4043: 4042: 4036: 4034: 4030: 4029: 4022: 4021: 4014: 4007: 3999: 3993: 3992: 3987: 3978: 3977: 3966: 3965:External links 3963: 3962: 3961: 3915:(3): 150–155, 3901: 3890:Rubin, Jean E. 3887: 3854: 3847: 3840: 3829: 3813: 3806:David K. Lewis 3803: 3793: 3786:John L. Kelley 3781: 3778: 3776: 3775: 3766: 3755:locus citandum 3746: 3736: 3734: 3731: 3727:proper classes 3659: 3656: 3653: 3650: 3647: 3644: 3606: 3603: 3600: 3597: 3594: 3591: 3520: 3501: 3498: 3485:inductive set 3464: 3461: 3458: 3455: 3435: 3432: 3429: 3426: 3423: 3420: 3417: 3397: 3394: 3391: 3350:The import of 3337: 3334: 3331: 3328: 3325: 3322: 3294: 3291: 3288: 3231:The import of 3217: 3214: 3179:The import of 3109: 3106: 3103: 3100: 3097: 3061: 3058: 3055: 3034:The import of 3020: 3017: 3014: 2934:The import of 2921: 2918: 2915: 2912: 2892: 2889: 2886: 2827: 2826: 2815: 2812: 2792: 2772: 2769: 2766: 2763: 2760: 2757: 2754: 2734: 2697:Extensionality 2682: 2679: 2676: 2673: 2653: 2650: 2647: 2605: 2602: 2599: 2570: 2569: 2545: 2542: 2539: 2536: 2533: 2530: 2527: 2524: 2521: 2503: 2489: 2469: 2463: 2407: 2404: 2403: 2402: 2398: 2387: 2382: 2374: 2368: 2360: 2338: 2337:) denote the Δ 2310: 2307: 2260: 2240: 2237: 2213: 2210: 2128:proper classes 2100: 2097: 2096: 2095: 2089: 2083: 2042: 2041: 2030: 2027: 2024: 2021: 2018: 2015: 2012: 2009: 2006: 2003: 2000: 1997: 1994: 1991: 1988: 1985: 1982: 1979: 1976: 1973: 1970: 1967: 1964: 1961: 1958: 1955: 1952: 1949: 1946: 1943: 1940: 1937: 1934: 1931: 1928: 1925: 1922: 1919: 1916: 1913: 1910: 1907: 1904: 1901: 1898: 1875: 1872: 1869: 1866: 1863: 1860: 1823: 1822: 1811: 1808: 1805: 1802: 1799: 1796: 1793: 1790: 1787: 1784: 1781: 1778: 1775: 1772: 1769: 1766: 1762: 1759: 1756: 1753: 1750: 1747: 1744: 1740: 1737: 1734: 1731: 1728: 1725: 1722: 1718: 1715: 1711: 1708: 1669: 1666: 1663: 1660: 1643: 1642: 1631: 1628: 1625: 1622: 1619: 1616: 1613: 1610: 1607: 1604: 1601: 1598: 1595: 1592: 1589: 1586: 1582: 1579: 1576: 1573: 1570: 1567: 1564: 1560: 1557: 1554: 1551: 1548: 1545: 1542: 1538: 1535: 1531: 1528: 1477: 1476: 1461: 1458: 1455: 1452: 1449: 1446: 1443: 1440: 1437: 1434: 1431: 1428: 1425: 1422: 1419: 1416: 1413: 1410: 1407: 1404: 1401: 1398: 1395: 1392: 1389: 1386: 1383: 1380: 1377: 1374: 1371: 1368: 1364: 1363: 1360: 1357: 1354: 1351: 1348: 1345: 1342: 1339: 1336: 1333: 1330: 1327: 1324: 1321: 1318: 1315: 1312: 1309: 1306: 1303: 1300: 1297: 1294: 1291: 1288: 1285: 1282: 1279: 1276: 1273: 1270: 1267: 1264: 1261: 1260: 1199: 1196: 1193: 1190: 1187: 1184: 1181: 1178: 1175: 1172: 1169: 1146: 1143: 1140: 1137: 1134: 1119: 1118: 1107: 1104: 1101: 1098: 1095: 1092: 1089: 1086: 1082: 1078: 1075: 1072: 1069: 1066: 1063: 1060: 1057: 1054: 1050: 1047: 1044: 1041: 1038: 1035: 1031: 1028: 1025: 1022: 1019: 1016: 1013: 1010: 1007: 1004: 1001: 997: 994: 990: 987: 956: 953: 950: 947: 944: 941: 938: 913: 912: 901: 898: 895: 892: 889: 886: 883: 878: 874: 870: 867: 864: 861: 856: 852: 848: 845: 842: 839: 836: 833: 830: 827: 824: 821: 818: 815: 812: 809: 804: 800: 796: 793: 790: 785: 781: 777: 750: 747: 744: 741: 714: 711: 708: 705: 702: 699: 696: 693: 690: 687: 639: 638: 627: 624: 621: 618: 615: 612: 609: 606: 603: 600: 597: 594: 591: 588: 585: 582: 579: 576: 573: 570: 567: 564: 561: 558: 555: 552: 549: 546: 515: 514: 503: 500: 497: 494: 491: 488: 485: 482: 479: 476: 473: 470: 467: 464: 461: 457: 454: 451: 447: 444: 440: 437: 421:Extensionality 417: 416: 397: 394: 391: 388: 385: 382: 379: 376: 373: 370: 367: 348: 337: 334: 331: 328: 325: 322: 319: 316: 307:is defined by 296: 282: 271: 268: 265: 262: 259: 256: 253: 250: 226: 223: 220: 200: 146: 143: 119:Thoralf Skolem 99:John L. Kelley 84:proper classes 15: 9: 6: 4: 3: 2: 6435: 6424: 6421: 6420: 6418: 6405: 6404: 6399: 6391: 6385: 6382: 6380: 6377: 6375: 6372: 6370: 6367: 6363: 6360: 6359: 6358: 6355: 6353: 6350: 6348: 6345: 6343: 6339: 6336: 6334: 6331: 6329: 6326: 6324: 6321: 6319: 6316: 6315: 6313: 6309: 6303: 6300: 6298: 6295: 6293: 6292:Recursive set 6290: 6288: 6285: 6283: 6280: 6278: 6275: 6273: 6270: 6266: 6263: 6261: 6258: 6256: 6253: 6251: 6248: 6246: 6243: 6242: 6241: 6238: 6236: 6233: 6231: 6228: 6226: 6223: 6221: 6218: 6216: 6213: 6212: 6210: 6208: 6204: 6198: 6195: 6193: 6190: 6188: 6185: 6183: 6180: 6178: 6175: 6173: 6170: 6168: 6165: 6161: 6158: 6156: 6153: 6151: 6148: 6147: 6146: 6143: 6141: 6138: 6136: 6133: 6131: 6128: 6126: 6123: 6121: 6118: 6114: 6111: 6110: 6109: 6106: 6102: 6101:of arithmetic 6099: 6098: 6097: 6094: 6090: 6087: 6085: 6082: 6080: 6077: 6075: 6072: 6070: 6067: 6066: 6065: 6062: 6058: 6055: 6053: 6050: 6049: 6048: 6045: 6044: 6042: 6040: 6036: 6030: 6027: 6025: 6022: 6020: 6017: 6015: 6012: 6009: 6008:from ZFC 6005: 6002: 6000: 5997: 5991: 5988: 5987: 5986: 5983: 5981: 5978: 5976: 5973: 5972: 5971: 5968: 5966: 5963: 5961: 5958: 5956: 5953: 5951: 5948: 5946: 5943: 5941: 5938: 5937: 5935: 5933: 5929: 5919: 5918: 5914: 5913: 5908: 5907:non-Euclidean 5905: 5901: 5898: 5896: 5893: 5891: 5890: 5886: 5885: 5883: 5880: 5879: 5877: 5873: 5869: 5866: 5864: 5861: 5860: 5859: 5855: 5851: 5848: 5847: 5846: 5842: 5838: 5835: 5833: 5830: 5828: 5825: 5823: 5820: 5818: 5815: 5813: 5810: 5809: 5807: 5803: 5802: 5800: 5795: 5789: 5784:Example  5781: 5773: 5768: 5767: 5766: 5763: 5761: 5758: 5754: 5751: 5749: 5746: 5744: 5741: 5739: 5736: 5735: 5734: 5731: 5729: 5726: 5724: 5721: 5719: 5716: 5712: 5709: 5707: 5704: 5703: 5702: 5699: 5695: 5692: 5690: 5687: 5685: 5682: 5680: 5677: 5676: 5675: 5672: 5670: 5667: 5663: 5660: 5658: 5655: 5653: 5650: 5649: 5648: 5645: 5641: 5638: 5636: 5633: 5631: 5628: 5626: 5623: 5621: 5618: 5616: 5613: 5612: 5611: 5608: 5606: 5603: 5601: 5598: 5596: 5593: 5589: 5586: 5584: 5581: 5579: 5576: 5574: 5571: 5570: 5569: 5566: 5564: 5561: 5559: 5556: 5554: 5551: 5547: 5544: 5542: 5541:by definition 5539: 5538: 5537: 5534: 5530: 5527: 5526: 5525: 5522: 5520: 5517: 5515: 5512: 5510: 5507: 5505: 5502: 5501: 5498: 5495: 5493: 5489: 5484: 5478: 5474: 5464: 5461: 5459: 5456: 5454: 5451: 5449: 5446: 5444: 5441: 5439: 5436: 5434: 5431: 5429: 5428:Kripke–Platek 5426: 5424: 5421: 5417: 5414: 5412: 5409: 5408: 5407: 5404: 5403: 5401: 5397: 5389: 5386: 5385: 5384: 5381: 5379: 5376: 5372: 5369: 5368: 5367: 5364: 5362: 5359: 5357: 5354: 5352: 5349: 5347: 5344: 5341: 5337: 5333: 5330: 5326: 5323: 5321: 5318: 5316: 5313: 5312: 5311: 5307: 5304: 5303: 5301: 5299: 5295: 5291: 5283: 5280: 5278: 5275: 5273: 5272:constructible 5270: 5269: 5268: 5265: 5263: 5260: 5258: 5255: 5253: 5250: 5248: 5245: 5243: 5240: 5238: 5235: 5233: 5230: 5228: 5225: 5223: 5220: 5218: 5215: 5213: 5210: 5208: 5205: 5204: 5202: 5200: 5195: 5187: 5184: 5182: 5179: 5177: 5174: 5172: 5169: 5167: 5164: 5162: 5159: 5158: 5156: 5152: 5149: 5147: 5144: 5143: 5142: 5139: 5137: 5134: 5132: 5129: 5127: 5124: 5122: 5118: 5114: 5112: 5109: 5105: 5102: 5101: 5100: 5097: 5096: 5093: 5090: 5088: 5084: 5074: 5071: 5069: 5066: 5064: 5061: 5059: 5056: 5054: 5051: 5049: 5046: 5042: 5039: 5038: 5037: 5034: 5030: 5025: 5024: 5023: 5020: 5019: 5017: 5015: 5011: 5003: 5000: 4998: 4995: 4993: 4990: 4989: 4988: 4985: 4983: 4980: 4978: 4975: 4973: 4970: 4968: 4965: 4963: 4960: 4958: 4955: 4954: 4952: 4950: 4949:Propositional 4946: 4940: 4937: 4935: 4932: 4930: 4927: 4925: 4922: 4920: 4917: 4915: 4912: 4908: 4905: 4904: 4903: 4900: 4898: 4895: 4893: 4890: 4888: 4885: 4883: 4880: 4878: 4877:Logical truth 4875: 4873: 4870: 4869: 4867: 4865: 4861: 4858: 4856: 4852: 4846: 4843: 4841: 4838: 4836: 4833: 4831: 4828: 4826: 4823: 4821: 4817: 4813: 4809: 4807: 4804: 4802: 4799: 4797: 4793: 4790: 4789: 4787: 4785: 4779: 4774: 4768: 4765: 4763: 4760: 4758: 4755: 4753: 4750: 4748: 4745: 4743: 4740: 4738: 4735: 4733: 4730: 4728: 4725: 4723: 4720: 4718: 4715: 4713: 4710: 4706: 4703: 4702: 4701: 4698: 4697: 4695: 4691: 4687: 4680: 4675: 4673: 4668: 4666: 4661: 4660: 4657: 4645: 4644:Ernst Zermelo 4642: 4640: 4637: 4635: 4632: 4630: 4629:Willard Quine 4627: 4625: 4622: 4620: 4617: 4615: 4612: 4610: 4607: 4605: 4602: 4600: 4597: 4595: 4592: 4590: 4587: 4586: 4584: 4582: 4581:Set theorists 4578: 4572: 4569: 4567: 4564: 4562: 4559: 4558: 4556: 4550: 4548: 4545: 4544: 4541: 4533: 4530: 4528: 4527:Kripke–Platek 4525: 4521: 4518: 4517: 4516: 4513: 4512: 4511: 4508: 4504: 4501: 4500: 4499: 4498: 4494: 4490: 4487: 4486: 4485: 4482: 4481: 4478: 4475: 4473: 4470: 4468: 4465: 4463: 4460: 4459: 4457: 4453: 4447: 4444: 4442: 4439: 4437: 4434: 4432: 4430: 4425: 4423: 4420: 4418: 4415: 4412: 4408: 4405: 4403: 4400: 4396: 4393: 4391: 4388: 4386: 4383: 4382: 4381: 4378: 4375: 4371: 4368: 4366: 4363: 4361: 4358: 4356: 4353: 4352: 4350: 4347: 4343: 4337: 4334: 4332: 4329: 4327: 4324: 4322: 4319: 4317: 4314: 4312: 4309: 4307: 4304: 4300: 4297: 4295: 4292: 4291: 4290: 4287: 4285: 4282: 4280: 4277: 4275: 4272: 4270: 4267: 4264: 4260: 4257: 4255: 4252: 4250: 4247: 4246: 4244: 4238: 4235: 4234: 4231: 4225: 4222: 4220: 4217: 4215: 4212: 4210: 4207: 4205: 4202: 4200: 4197: 4195: 4192: 4189: 4186: 4184: 4181: 4180: 4178: 4176: 4172: 4164: 4163:specification 4161: 4159: 4156: 4155: 4154: 4151: 4150: 4147: 4144: 4142: 4139: 4137: 4134: 4132: 4129: 4127: 4124: 4122: 4119: 4117: 4114: 4112: 4109: 4105: 4102: 4101: 4100: 4097: 4095: 4092: 4088: 4085: 4083: 4080: 4078: 4075: 4074: 4073: 4070: 4068: 4065: 4064: 4062: 4060: 4056: 4051: 4041: 4038: 4037: 4035: 4031: 4027: 4020: 4015: 4013: 4008: 4006: 4001: 4000: 3997: 3991: 3988: 3986: 3983: 3982: 3981: 3976: 3974: 3969: 3968: 3958: 3954: 3949: 3944: 3940: 3936: 3932: 3928: 3923: 3918: 3914: 3910: 3906: 3902: 3899: 3895: 3891: 3888: 3883: 3878: 3874: 3870: 3863: 3859: 3855: 3852: 3848: 3845: 3841: 3838: 3832: 3830:0-534-06624-0 3826: 3822: 3818: 3814: 3811: 3807: 3804: 3801: 3797: 3796:Lemmon, E. J. 3794: 3791: 3787: 3784: 3783: 3770: 3763: 3760: 3756: 3750: 3741: 3737: 3730: 3728: 3724: 3720: 3716: 3712: 3708: 3703: 3701: 3697: 3693: 3689: 3685: 3683: 3679: 3675: 3671: 3657: 3645: 3642: 3634: 3630: 3626: 3624: 3620: 3604: 3601: 3595: 3589: 3581: 3577: 3573: 3570: 3566: 3564: 3560: 3556: 3552: 3548: 3544: 3540: 3536: 3534: 3518: 3499: 3496: 3488: 3484: 3480: 3475: 3462: 3459: 3456: 3453: 3433: 3430: 3424: 3418: 3415: 3395: 3392: 3389: 3381: 3377: 3373: 3371: 3367: 3363: 3359: 3357: 3353: 3348: 3335: 3329: 3326: 3323: 3320: 3312: 3308: 3289: 3286: 3278: 3274: 3272: 3268: 3264: 3260: 3256: 3252: 3248: 3246: 3242: 3238: 3234: 3229: 3215: 3212: 3204: 3200: 3196: 3194: 3190: 3186: 3182: 3177: 3175: 3171: 3167: 3163: 3159: 3157: 3153: 3149: 3145: 3141: 3137: 3136:ordered pairs 3133: 3129: 3127: 3123: 3104: 3101: 3098: 3088:implies that 3087: 3083: 3079: 3075: 3056: 3045: 3041: 3037: 3032: 3018: 3015: 3012: 3004: 3000: 2996: 2992: 2990: 2986: 2982: 2978: 2974: 2972: 2968: 2964: 2960: 2956: 2952: 2949: 2945: 2941: 2937: 2932: 2919: 2916: 2913: 2910: 2890: 2887: 2884: 2876: 2872: 2868: 2864: 2863:III. Subsets: 2860: 2858: 2854: 2850: 2846: 2842: 2840: 2836: 2832: 2813: 2810: 2803:is a set and 2790: 2767: 2764: 2761: 2755: 2752: 2732: 2724: 2723: 2722: 2720: 2716: 2714: 2710: 2706: 2702: 2698: 2695:Identical to 2693: 2680: 2677: 2674: 2671: 2651: 2648: 2645: 2637: 2633: 2629: 2625: 2621: 2617: 2603: 2600: 2597: 2589: 2585: 2581: 2577: 2574: 2567: 2563: 2559: 2543: 2534: 2528: 2525: 2522: 2508: 2504: 2501: 2497: 2493: 2490: 2487: 2483: 2482: 2481: 2478: 2476: 2468: 2462: 2460: 2456: 2452: 2451:Impredicative 2448: 2444: 2440: 2437: 2433: 2432:impredicative 2429: 2425: 2421: 2417: 2413: 2397: 2393: 2386: 2383: 2380: 2373: 2369: 2366: 2359: 2356: 2355: 2354: 2352: 2348: 2344: 2336: 2332: 2328: 2324: 2320: 2316: 2306: 2304: 2300: 2296: 2291: 2289: 2285: 2281: 2277: 2274:The class of 2272: 2258: 2238: 2235: 2227: 2211: 2208: 2201: 2200:limit ordinal 2197: 2193: 2189: 2187: 2182: 2178: 2176: 2172: 2168: 2164: 2160: 2156: 2151: 2149: 2145: 2141: 2137: 2133: 2132:impredicative 2129: 2125: 2120: 2118: 2114: 2110: 2106: 2093: 2090: 2087: 2084: 2081: 2077: 2074: 2073: 2072: 2069: 2067: 2063: 2059: 2055: 2051: 2047: 2028: 2010: 2007: 2004: 2001: 1998: 1995: 1992: 1983: 1980: 1977: 1971: 1965: 1962: 1959: 1956: 1950: 1941: 1938: 1935: 1929: 1923: 1920: 1917: 1911: 1908: 1905: 1899: 1889: 1888: 1887: 1873: 1867: 1861: 1858: 1851:, then so is 1850: 1846: 1842: 1838: 1834: 1830: 1828: 1809: 1803: 1800: 1785: 1782: 1779: 1776: 1773: 1770: 1767: 1760: 1751: 1748: 1745: 1738: 1732: 1729: 1726: 1716: 1709: 1699: 1698: 1697: 1695: 1691: 1687: 1684:, namely the 1683: 1667: 1664: 1661: 1658: 1650: 1648: 1629: 1623: 1620: 1605: 1602: 1599: 1593: 1590: 1587: 1580: 1571: 1568: 1565: 1558: 1552: 1549: 1546: 1536: 1529: 1519: 1518: 1517: 1515: 1511: 1507: 1503: 1499: 1497: 1492: 1490: 1486: 1482: 1459: 1447: 1444: 1441: 1432: 1429: 1423: 1420: 1417: 1411: 1408: 1405: 1399: 1396: 1393: 1381: 1375: 1369: 1358: 1349: 1346: 1340: 1337: 1334: 1328: 1325: 1322: 1319: 1313: 1304: 1301: 1295: 1286: 1277: 1274: 1265: 1251: 1250: 1249: 1247: 1243: 1239: 1238: 1233: 1229: 1226: 1224: 1219: 1217: 1213: 1191: 1188: 1185: 1179: 1173: 1141: 1138: 1135: 1124: 1105: 1090: 1087: 1084: 1080: 1076: 1073: 1070: 1061: 1058: 1055: 1048: 1042: 1039: 1036: 1029: 1017: 1014: 1011: 1008: 1005: 995: 988: 978: 977: 976: 974: 970: 951: 948: 945: 939: 936: 928: 924: 921:For any sets 920: 918: 899: 890: 887: 884: 876: 872: 868: 865: 862: 859: 854: 850: 846: 843: 837: 828: 825: 822: 816: 810: 802: 798: 794: 791: 788: 783: 779: 768: 767: 766: 764: 745: 739: 731: 728: 706: 700: 697: 694: 688: 685: 678: 674: 673: 667: 663: 659: 655: 654:free variable 651: 647: 643: 625: 613: 610: 607: 601: 598: 595: 589: 583: 580: 577: 574: 568: 556: 553: 547: 537: 536: 535: 533: 529: 525: 523: 518: 501: 495: 492: 489: 480: 477: 474: 468: 465: 462: 455: 445: 438: 428: 427: 426: 424: 422: 414: 410: 395: 389: 386: 383: 377: 374: 368: 357: 353: 349: 335: 326: 323: 317: 294: 287: 283: 269: 263: 260: 257: 251: 240: 224: 221: 218: 208: 205: 201: 198: 194: 190: 186: 185: 184: 182: 177: 175: 171: 167: 163: 159: 155: 151: 142: 139: 135: 131: 126: 124: 120: 116: 112: 108: 104: 103:Anthony Morse 100: 95: 93: 89: 85: 81: 77: 73: 69: 65: 62: 58: 54: 50: 46: 42: 38: 34: 30: 26: 22: 6394: 6192:Ultraproduct 6039:Model theory 6004:Independence 5940:Formal proof 5932:Proof theory 5915: 5888: 5845:real numbers 5817:second-order 5728:Substitution 5605:Metalanguage 5546:conservative 5519:Axiom schema 5463:Constructive 5433:Morse–Kelley 5432: 5399:Set theories 5378:Aleph number 5371:inaccessible 5277:Grothendieck 5161:intersection 5048:Higher-order 5036:Second-order 4982:Truth tables 4939:Venn diagram 4722:Formal proof 4594:Georg Cantor 4589:Paul Bernays 4520:Morse–Kelley 4519: 4495: 4428: 4427:Subset  4374:hereditarily 4336:Venn diagram 4294:ordered pair 4209:Intersection 4153:Axiom schema 3979: 3972: 3912: 3908: 3893: 3872: 3868: 3850: 3843: 3820: 3809: 3799: 3789: 3788:1975 (1955) 3769: 3761: 3754: 3749: 3740: 3722: 3718: 3714: 3710: 3706: 3704: 3687: 3686: 3681: 3673: 3672: 3632: 3628: 3627: 3622: 3618: 3579: 3575: 3571: 3568: 3567: 3563:real numbers 3551:Peano axioms 3546: 3538: 3537: 3532: 3486: 3478: 3476: 3382:, such that 3379: 3375: 3374: 3361: 3360: 3355: 3351: 3349: 3310: 3306: 3276: 3275: 3271:order theory 3250: 3249: 3244: 3240: 3236: 3232: 3230: 3202: 3198: 3197: 3180: 3178: 3173: 3169: 3165: 3161: 3160: 3131: 3130: 3125: 3121: 3120:is a set if 3085: 3081: 3077: 3073: 3043: 3039: 3035: 3033: 3002: 2998: 2994: 2993: 2980: 2976: 2975: 2970: 2966: 2962: 2958: 2957:, the class 2954: 2950: 2947: 2943: 2939: 2935: 2933: 2874: 2870: 2866: 2862: 2861: 2856: 2844: 2843: 2838: 2834: 2830: 2828: 2718: 2717: 2712: 2700: 2696: 2694: 2635: 2631: 2627: 2623: 2619: 2618: 2587: 2584:proper class 2579: 2575: 2572: 2571: 2565: 2561: 2557: 2509:of the form 2499: 2495: 2491: 2485: 2479: 2474: 2471: 2466: 2423: 2419: 2416:J. L. Kelley 2409: 2395: 2384: 2371: 2363:is model of 2357: 2346: 2334: 2330: 2314: 2312: 2309:Model theory 2292: 2287: 2283: 2280:well-ordered 2273: 2225: 2195: 2185: 2179: 2152: 2143: 2135: 2121: 2102: 2070: 2065: 2061: 2057: 2053: 2049: 2045: 2043: 1848: 1844: 1840: 1832: 1825: 1824: 1693: 1689: 1681: 1645: 1644: 1513: 1509: 1501: 1494: 1493: 1484: 1478: 1245: 1235: 1232:proper class 1227: 1221: 1220: 1123:ordered pair 1120: 972: 968: 926: 922: 915: 914: 762: 729: 726: 676: 669: 665: 661: 657: 649: 645: 641: 640: 527: 520: 519: 516: 419: 418: 411:is also the 408: 351: 238: 193:proper class 188: 178: 170:proper class 160:consists of 148: 127: 122: 110: 96: 76:axiom schema 56: 52: 48: 44: 40: 36: 32: 28: 24: 18: 6302:Type theory 6250:undecidable 6182:Truth value 6069:equivalence 5748:non-logical 5361:Enumeration 5351:Isomorphism 5298:cardinality 5282:Von Neumann 5247:Ultrafilter 5212:Uncountable 5146:equivalence 5063:Quantifiers 5053:Fixed-point 5022:First-order 4902:Consistency 4887:Proposition 4864:Traditional 4835:Lindström's 4825:Compactness 4767:Type theory 4712:Cardinality 4619:Thomas Jech 4462:Alternative 4441:Uncountable 4395:Ultrafilter 4254:Cardinality 4158:replacement 4099:Determinacy 3875:: 111–124, 3692:Equivalents 3629:IX. Choice: 3569:Definition: 3354:is that of 3235:is that of 3038:is that of 2938:is that of 2573:Definition: 2560:satisfying 2457:(1951) and 2412:Wang (1949) 2299:syntactical 2188:being a set 2140:predicative 1508:of the set 107:Wang (1949) 61:first-order 6113:elementary 5806:arithmetic 5674:Quantifier 5652:functional 5524:Expression 5242:Transitive 5186:identities 5171:complement 5104:hereditary 5087:Set theory 4614:Kurt Gödel 4599:Paul Cohen 4436:Transitive 4204:Identities 4188:Complement 4175:Operations 4136:Regularity 4104:projective 4067:Adjunction 4026:Set theory 3898:urelements 3780:References 3549:is a set, 3356:Foundation 3313:such that 3263:surjection 3228:is a set. 3176:is a set. 3128:are sets. 3031:is a set. 2995:IV. Union: 2991:provable. 2989:Separation 2985:singletons 2973:is a set. 2946:: for any 2853:null class 2847:: Boolean 2620:I. Extent: 2418:'s (1955) 2341:definable 2325:. Let the 2109:urelements 2099:Discussion 2044:Note that 1843:; (ii) if 1696:is a set. 1516:is a set. 732:such that 522:Foundation 350:The class 6384:Supertask 6287:Recursion 6245:decidable 6079:saturated 6057:of models 5980:deductive 5975:axiomatic 5895:Hilbert's 5882:Euclidean 5863:canonical 5786:axiomatic 5718:Signature 5647:Predicate 5536:Extension 5458:Ackermann 5383:Operation 5262:Universal 5252:Recursive 5227:Singleton 5222:Inhabited 5207:Countable 5197:Types of 5181:power set 5151:partition 5068:Predicate 5014:Predicate 4929:Syllogism 4919:Soundness 4892:Inference 4882:Tautology 4784:paradoxes 4547:Paradoxes 4467:Axiomatic 4446:Universal 4422:Singleton 4417:Recursive 4360:Countable 4355:Amorphous 4214:Power set 4131:Power set 4082:dependent 4077:countable 3971:Download 3905:Wang, Hao 3652:∅ 3646:− 3602:∈ 3519:∅ 3497:∅ 3457:∈ 3446:whenever 3431:∈ 3419:∪ 3393:∈ 3390:∅ 3333:∅ 3324:∩ 3293:∅ 3290:≠ 3267:bijection 3259:injection 3213:⋃ 3144:functions 3140:relations 3072:of a set 3016:∪ 2940:Power Set 2914:∈ 2888:⊆ 2791:β 2762:α 2756:∈ 2753:β 2733:β 2725:For each 2675:∈ 2649:∈ 2626:and each 2622:For each 2601:∈ 2455:Mostowski 2392:power set 2353:). Then: 2303:semantics 2259:ω 2236:ω 2209:ω 2107:includes 2008:∈ 2002:∨ 1987:↔ 1981:∈ 1969:∀ 1966:∧ 1960:∈ 1948:∃ 1945:→ 1939:∈ 1927:∀ 1924:∧ 1918:∈ 1915:∅ 1912:∧ 1897:∃ 1862:∪ 1837:empty set 1798:→ 1783:∈ 1777:∧ 1771:∈ 1758:∃ 1755:↔ 1749:∈ 1736:∀ 1733:∧ 1714:∀ 1707:∀ 1665:⋃ 1618:→ 1603:∈ 1597:→ 1591:∈ 1578:∀ 1575:↔ 1569:∈ 1556:∀ 1553:∧ 1534:∀ 1527:∀ 1496:Power set 1439:→ 1430:∈ 1427:⟩ 1415:⟨ 1412:∧ 1406:∈ 1403:⟩ 1391:⟨ 1379:∀ 1373:∀ 1367:∀ 1359:∧ 1347:∈ 1344:⟩ 1332:⟨ 1329:∧ 1323:∈ 1311:∃ 1308:→ 1293:∀ 1284:∃ 1281:↔ 1272:¬ 1263:∀ 1216:functions 1212:relations 1145:⟩ 1133:⟨ 1081:∨ 1065:↔ 1059:∈ 1046:∀ 1043:∧ 1027:∃ 1024:→ 1012:∧ 993:∀ 986:∀ 885:∧ 838:ϕ 832:↔ 826:∈ 814:∀ 808:∃ 776:∀ 740:ϕ 701:ϕ 698:∣ 605:→ 599:∈ 587:∀ 584:∧ 578:∈ 566:∃ 563:→ 560:∅ 545:∀ 487:→ 478:∈ 472:↔ 466:∈ 453:∀ 443:∀ 436:∀ 387:∈ 381:→ 366:∀ 330:∅ 315:∀ 295:∅ 286:empty set 261:∈ 249:∃ 207:predicate 55:) or the 6417:Category 6369:Logicism 6362:timeline 6338:Concrete 6197:Validity 6167:T-schema 6160:Kripke's 6155:Tarski's 6150:semantic 6140:Strength 6089:submodel 6084:spectrum 6052:function 5900:Tarski's 5889:Elements 5876:geometry 5832:Robinson 5753:variable 5738:function 5711:spectrum 5701:Sentence 5657:variable 5600:Language 5553:Relation 5514:Automata 5504:Alphabet 5488:language 5342:-jection 5320:codomain 5306:Function 5267:Universe 5237:Infinite 5141:Relation 4924:Validity 4914:Argument 4812:theorem, 4551:Problems 4455:Theories 4431:Superset 4407:Infinite 4236:Concepts 4116:Infinity 4033:Overview 3957:16588874 3860:(1950), 3819:(1987). 3623:domain c 3555:integers 3483:infinite 3170:domain f 3084:). Then 2492:domain f 2461:(1991). 2286:. Hence 2105:ontology 1827:Infinity 611:∉ 557:≠ 532:disjoint 327:∉ 197:mnemonic 154:ontology 125:(1965). 115:topology 6311:Related 6108:Diagram 6006: ( 5985:Hilbert 5970:Systems 5965:Theorem 5843:of the 5788:systems 5568:Formula 5563:Grammar 5479: ( 5423:General 5136:Forcing 5121:Element 5041:Monadic 4816:paradox 4757:Theorem 4693:General 4489:General 4484:Zermelo 4390:subbase 4372: ( 4311:Forcing 4289:Element 4261: ( 4239:Methods 4126:Pairing 3948:1062986 3939:0029850 3892:(1967) 3808:(1991) 3798:(1986) 3759:Quine's 3688:Develop 3684:above. 3539:Develop 3362:Develop 3358:above. 3251:Develop 3239:above. 3174:range f 3132:Develop 3040:Pairing 2977:Develop 2903:, then 2845:Develop 2699:above. 2496:range f 2484:He did 2477:below. 2475:Develop 2436:Quine's 2406:History 2343:subsets 2278:can be 1692:. Then 1512:. Then 1506:subsets 1240:can be 917:Pairing 204:monadic 162:classes 19:In the 6074:finite 5837:Skolem 5790:  5765:Theory 5733:Symbol 5723:String 5706:atomic 5583:ground 5578:closed 5573:atomic 5529:ground 5492:syntax 5388:binary 5315:domain 5232:Finite 4997:finite 4855:Logics 4814:  4762:Theory 4380:Filter 4370:Finite 4306:Family 4249:Almost 4087:global 4072:Choice 4059:Axioms 3955:  3945:  3937:  3929:  3827:  3148:domain 2517:  2390:, the 2317:, the 2167:domain 1165:  644:Let φ( 354:, the 216:  156:. The 6064:Model 5812:Peano 5669:Proof 5509:Arity 5438:Naive 5325:image 5257:Fuzzy 5217:Empty 5166:union 5111:Class 4752:Model 4742:Lemma 4700:Axiom 4472:Naive 4402:Fuzzy 4365:Empty 4348:types 4299:tuple 4269:Class 4263:large 4224:Union 4141:Union 3931:88430 3927:JSTOR 3865:(PDF) 3733:Notes 3574:is a 3237:Union 3152:range 2948:class 2877:, if 2578:is a 2459:Lewis 2349:(see 2183:plus 2171:range 1686:union 1647:Union 1244:into 1230:is a 677:class 652:is a 88:Quine 59:is a 6187:Type 5990:list 5794:list 5771:list 5760:Term 5694:rank 5588:open 5482:list 5294:Maps 5199:sets 5058:Free 5028:list 4778:list 4705:list 4385:base 3953:PMID 3825:ISBN 3753:The 3479:VIII 3408:and 3243:and 3191:and 3124:and 3001:and 2494:and 2370:Def( 2115:and 2078:and 2064:and 2056:and 2048:and 1651:Let 1500:Let 1214:and 971:and 925:and 727:sets 656:and 284:The 202:The 166:sets 101:and 5874:of 5856:of 5804:of 5336:Sur 5310:Map 5117:Ur- 5099:Set 4346:Set 3943:PMC 3917:doi 3877:doi 3837:NBG 3707:III 3696:ZFC 3621:of 3578:if 3541:: 3352:VII 3309:of 3279:If 3201:If 3193:ZFC 3189:NBG 3187:in 3164:If 3082:III 2997:If 2967:III 2944:III 2936:III 2865:If 2709:ZFC 2707:in 2632:x=y 2580:set 2568:)." 2486:not 2447:ZFC 2394:of 2388:κ+1 2379:NBG 2365:ZFC 2345:of 2323:ZFC 2321:in 2224:as 2117:NBG 2113:ZFC 2092:NBG 2086:NBG 2080:NBG 2076:ZFC 672:NBG 530:is 181:NBG 150:NBG 132:of 78:of 47:), 39:), 31:), 6419:: 6260:NP 5884:: 5878:: 5808:: 5485:), 5340:Bi 5332:In 3951:, 3941:, 3935:MR 3933:, 3925:, 3913:35 3911:, 3873:37 3871:, 3867:, 3723:MK 3719:MK 3715:IV 3711:IV 3690:: 3674:IX 3670:. 3625:. 3565:. 3561:, 3557:, 3553:, 3545:, 3372:. 3368:, 3364:: 3273:. 3269:, 3265:, 3261:, 3257:, 3253:: 3245:VI 3241:IV 3233:VI 3195:. 3158:. 3154:, 3150:, 3146:, 3142:, 3138:, 3086:IV 3044:IV 3036:IV 2987:. 2979:: 2859:. 2841:. 2745:, 2638:, 2630:, 2616:. 2590:, 2449:. 2439:ML 2177:. 2150:. 2119:. 1886:. 1491:. 1248:. 1125:, 975:. 94:. 92:ML 53:QM 45:MT 37:KM 29:MK 23:, 6340:/ 6255:P 6010:) 5796:) 5792:( 5689:∀ 5684:! 5679:∃ 5640:= 5635:↔ 5630:→ 5625:∧ 5620:∨ 5615:¬ 5338:/ 5334:/ 5308:/ 5119:) 5115:( 5002:∞ 4992:3 4780:) 4678:e 4671:t 4664:v 4429:· 4413:) 4409:( 4376:) 4265:) 4018:e 4011:t 4004:v 3960:. 3919:: 3900:. 3886:. 3879:: 3833:. 3658:. 3655:} 3649:{ 3643:V 3633:c 3619:x 3605:x 3599:) 3596:x 3593:( 3590:c 3580:c 3572:c 3547:N 3533:y 3500:. 3487:y 3463:. 3460:y 3454:x 3434:y 3428:} 3425:x 3422:{ 3416:x 3396:y 3380:y 3336:. 3330:= 3327:y 3321:x 3311:x 3307:y 3287:x 3216:x 3203:x 3181:V 3166:f 3126:y 3122:x 3108:} 3105:y 3102:, 3099:x 3096:{ 3078:x 3074:x 3060:} 3057:x 3054:{ 3019:y 3013:x 3003:y 2999:x 2981:V 2971:z 2963:y 2959:z 2955:x 2951:z 2920:. 2917:y 2911:z 2891:x 2885:z 2875:z 2871:y 2867:x 2857:V 2839:A 2835:B 2831:A 2814:, 2811:B 2771:} 2768:A 2765:: 2759:{ 2713:I 2701:I 2681:. 2678:y 2672:z 2652:x 2646:z 2636:z 2628:y 2624:x 2604:y 2598:x 2588:y 2576:x 2566:x 2564:( 2562:A 2558:x 2544:, 2541:} 2538:) 2535:x 2532:( 2529:A 2526:: 2523:x 2520:{ 2500:f 2399:κ 2396:V 2385:V 2375:κ 2372:V 2367:; 2361:κ 2358:V 2347:X 2339:0 2335:X 2331:V 2315:V 2288:V 2284:V 2239:. 2226:I 2212:, 2196:I 2186:I 2144:x 2136:x 2066:s 2062:p 2058:s 2054:p 2050:s 2046:p 2029:. 2026:] 2023:) 2020:] 2017:) 2014:] 2011:z 2005:w 1999:z 1996:= 1993:w 1990:[ 1984:x 1978:w 1975:( 1972:w 1963:y 1957:x 1954:[ 1951:x 1942:y 1936:z 1933:( 1930:z 1921:y 1909:y 1906:M 1903:[ 1900:y 1874:. 1871:} 1868:x 1865:{ 1859:x 1849:y 1845:x 1841:y 1833:y 1829:: 1810:. 1807:] 1804:s 1801:M 1795:) 1792:] 1789:) 1786:a 1780:y 1774:y 1768:x 1765:( 1761:y 1752:s 1746:x 1743:[ 1739:x 1730:a 1727:M 1724:( 1721:[ 1717:s 1710:a 1694:s 1690:a 1682:a 1668:a 1662:= 1659:s 1649:: 1630:. 1627:] 1624:p 1621:M 1615:) 1612:] 1609:) 1606:a 1600:y 1594:x 1588:y 1585:( 1581:y 1572:p 1566:x 1563:[ 1559:x 1550:a 1547:M 1544:( 1541:[ 1537:p 1530:a 1514:p 1510:a 1502:p 1498:: 1485:F 1460:. 1457:] 1454:) 1451:] 1448:y 1445:= 1442:x 1436:) 1433:F 1424:s 1421:, 1418:y 1409:F 1400:s 1397:, 1394:x 1388:( 1385:[ 1382:s 1376:y 1370:x 1356:] 1353:) 1350:F 1341:s 1338:, 1335:x 1326:C 1320:s 1317:( 1314:s 1305:x 1302:M 1299:[ 1296:x 1290:( 1287:F 1278:C 1275:M 1269:[ 1266:C 1246:C 1237:V 1228:C 1225:: 1198:} 1195:} 1192:y 1189:, 1186:x 1183:{ 1180:, 1177:} 1174:x 1171:{ 1168:{ 1142:y 1139:, 1136:x 1106:. 1103:] 1100:) 1097:] 1094:) 1091:y 1088:= 1085:s 1077:x 1074:= 1071:s 1068:( 1062:z 1056:s 1053:[ 1049:s 1040:z 1037:M 1034:( 1030:z 1021:) 1018:y 1015:M 1009:x 1006:M 1003:( 1000:[ 996:y 989:x 973:y 969:x 955:} 952:y 949:, 946:x 943:{ 940:= 937:z 927:y 923:x 919:: 900:. 897:] 894:) 891:x 888:M 882:) 877:n 873:W 869:. 866:. 863:. 860:, 855:1 851:W 847:, 844:x 841:( 835:( 829:Y 823:x 820:[ 817:x 811:Y 803:n 799:W 795:. 792:. 789:. 784:1 780:W 763:Y 749:) 746:x 743:( 730:x 713:} 710:) 707:x 704:( 695:x 692:{ 689:= 686:Y 666:x 662:x 658:Y 650:x 646:x 626:. 623:] 620:) 617:) 614:A 608:c 602:b 596:c 593:( 590:c 581:A 575:b 572:( 569:b 554:A 551:[ 548:A 528:A 524:: 502:. 499:) 496:Y 493:= 490:X 484:) 481:Y 475:z 469:X 463:z 460:( 456:z 450:( 446:Y 439:X 423:: 415:. 409:V 396:. 393:) 390:V 384:x 378:x 375:M 372:( 369:x 352:V 336:. 333:) 324:x 321:( 318:x 270:. 267:) 264:W 258:x 255:( 252:W 239:x 225:, 222:x 219:M 199:. 189:M 51:( 43:( 35:( 27:(

Index

foundations of mathematics
first-order
axiomatic set theory
von Neumann–Bernays–Gödel set theory
bound variables
axiom schema
Class Comprehension
proper classes
Quine
ML
John L. Kelley
Anthony Morse
Wang (1949)
topology
Thoralf Skolem
conservative extension
Zermelo–Fraenkel set theory
proper extension
NBG
ontology
universe of discourse
classes
sets
proper class
atomic sentences
NBG
proper class
mnemonic
monadic
predicate

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