New Foundations

The primitive predicates of Russellian unramified typed set theory (TST), a streamlined version of the theory of types, are equality (${\displaystyle =}$) and membership (${\displaystyle \in }$). TST has a linear hierarchy of types: type 0 consists of individuals otherwise undescribed. For each (meta-) natural number n, type n+1 objects are sets of type n objects; sets of type n have members of type n-1. Objects connected by identity must have the same type. The following two atomic formulas succinctly describe the typing rules: ${\displaystyle x^{n}=y^{n}\!}$ and ${\displaystyle x^{n}\in y^{n+1}}$. (Quinean set theory seeks to eliminate the need for such superscripts.)

The axioms of TST are:

• Extensionality: sets of the same (positive) type with the same members are equal;
• An axiom schema of comprehension, namely:

If ${\displaystyle \phi (x^{n})}$ is a formula, then the set ${\displaystyle \{x^{n}\mid \phi (x^{n})\}^{n+1}\!}$ exists.

In other words, given any formula ${\displaystyle \phi (x^{n})\!}$, the formula ${\displaystyle \exists A^{n+1}\forall x^{n}[x^{n}\in A^{n+1}\leftrightarrow \phi (x^{n})]}$ is an axiom where ${\displaystyle A^{n+1}\!}$ represents the set ${\displaystyle \{x^{n}\mid \phi (x^{n})\}^{n+1}\!}$ and is not free in ${\displaystyle \phi (x^{n})}$.

This type theory is much less complicated than the one first set out in the Principia Mathematica, which included types for relations whose arguments were not necessarily all of the same type. In 1914, Norbert Wiener showed how to code the ordered pair as a set of sets, making it possible to eliminate relation types in favor of the linear hierarchy of sets described here.

New Foundations can be finitely axiomatized.[4][5]

The category whose objects are the sets of NF and whose arrows are the functions between those sets is not Cartesian closed;[6] Cartesian closure can be a useful property for a category of sets. Since NF lacks Cartesian closure, not every function curries as one might intuitively expect, and NF is not a topos.

For many years, the outstanding problem with NF has been that it has not conclusively been proved to be relatively consistent to any other well-known axiomatic system in which arithmetic can be modelled. NF disproves Choice, and thus proves Infinity (Specker, 1953). But it is also known (Jensen, 1969) that allowing urelements (multiple distinct objects lacking members) yields NFU, a theory that is consistent relative to Peano arithmetic; if Infinity and Choice are added, the resulting theory has the same consistency strength as type theory with infinity or bounded Zermelo set theory. (NFU corresponds to a type theory TSTU, where type 0 has urelements, not just a single empty set.) There are other relatively consistent variants of NF.

NFU is, roughly speaking, weaker than NF because in NF, the power set of the universe is the universe itself, while in NFU, the power set of the universe may be strictly smaller than the universe (the power set of the universe contains only sets, while the universe may contain urelements). In fact, this is necessarily the case in NFU+"Choice".

Ernst Specker has shown that NF is equiconsistent with TST + Amb, where Amb is the axiom scheme of typical ambiguity which asserts ${\displaystyle \phi \leftrightarrow \phi ^{+}}$ for any formula ${\displaystyle \phi }$, ${\displaystyle \phi ^{+}}$ being the formula obtained by raising every type index in ${\displaystyle \phi }$ by one. NF is also equiconsistent with the theory TST augmented with a "type shifting automorphism", an operation which raises type by one, mapping each type onto the next higher type, and preserves equality and membership relations (and which cannot be used in instances of Comprehension: it is external to the theory). The same results hold for various fragments of TST in relation to the corresponding fragments of NF.

In the same year (1969) that Jensen proved NFU consistent, Grishin proved ${\displaystyle NF_{3}}$ consistent. ${\displaystyle NF_{3}}$ is the fragment of NF with full extensionality (no urelements) and those instances of Comprehension which can be stratified using just three types. This theory is a very awkward medium for mathematics (although there have been attempts to alleviate this awkwardness), largely because there is no obvious definition for an ordered pair. Despite this awkwardness, ${\displaystyle NF_{3}}$ is very interesting because every infinite model of TST restricted to three types satisfies Amb. Hence for every such model there is a model of ${\displaystyle NF_{3}}$ with the same theory. This does not hold for four types: ${\displaystyle NF_{4}}$ is the same theory as NF, and we have no idea how to obtain a model of TST with four types in which Amb holds.

In 1983, Marcel Crabbé proved consistent a system he called NFI, whose axioms are unrestricted extensionality and those instances of Comprehension in which no variable is assigned a type higher than that of the set asserted to exist. This is a predicativity restriction, though NFI is not a predicative theory: it admits enough impredicativity to define the set of natural numbers (defined as the intersection of all inductive sets; note that the inductive sets quantified over are of the same type as the set of natural numbers being defined). Crabbé also discussed a subtheory of NFI, in which only parameters (free variables) are allowed to have the type of the set asserted to exist by an instance of Comprehension. He called the result "predicative NF" (NFP); it is, of course, doubtful whether any theory with a self-membered universe is truly predicative. Holmes has shown that NFP has the same consistency strength as the predicative theory of types of Principia Mathematica without the Axiom of reducibility.

Since 2015, several candidate proofs by Randall Holmes of the consistency of NF relative to ZF have been available both on arxiv and on the logician's home page. Holmes demonstrates the equiconsistency of a 'weird' variant of TST, namely TTT_λ - 'tangled type theory with λ-types' - with NF. Holmes next shows that TTT_λ is consistent relative to ZFA, that is, ZF with atoms but without choice. Holmes demonstrates this by constructing in ZFA+C, that is, ZF with atoms and choice, a class model of ZFA which includes 'tangled webs of cardinals' . The candidate proofs are all rather long, but no irrecoverable faults have been identified by the NF community as yet.

NF steers clear of the three well-known paradoxes of set theory. That NFU, a consistent (relative to Peano arithmetic) theory, also avoids the paradoxes may increase one's confidence in this fact.

The Russell paradox: An easy matter; ${\displaystyle x\not \in x}$ is not a stratified formula, so the existence of ${\displaystyle \{x\mid x\not \in x\}}$ is not asserted by any instance of Comprehension. Quine said that he constructed NF with this paradox uppermost in mind.

Cantor's paradox of the largest cardinal number exploits the application of Cantor's theorem to the universal set. Cantor's theorem says (given ZFC) that the power set ${\displaystyle P(A)}$ of any set ${\displaystyle A}$ is larger than ${\displaystyle A}$ (there can be no injection (one-to-one map) from ${\displaystyle P(A)}$ into ${\displaystyle A}$). Now of course there is an injection from ${\displaystyle P(V)}$ into ${\displaystyle V}$, if ${\displaystyle V}$ is the universal set! The resolution requires that one observes that ${\displaystyle |A|<|P(A)|}$ makes no sense in the theory of types: the type of ${\displaystyle P(A)}$ is one higher than the type of ${\displaystyle A}$. The correctly typed version (which is a theorem in the theory of types for essentially the same reasons that the original form of Cantor's theorem works in ZF) is ${\displaystyle |P_{1}(A)|<|P(A)|}$, where ${\displaystyle P_{1}(A)}$ is the set of one-element subsets of ${\displaystyle A}$. The specific instance of this theorem of interest is ${\displaystyle |P_{1}(V)|<|P(V)|}$: there are fewer one-element sets than sets (and so fewer one-element sets than general objects, if we are in NFU). The "obvious" bijection ${\displaystyle x\mapsto \{x\}}$ from the universe to the one-element sets is not a set; it is not a set because its definition is unstratified. Note that in all known models of NFU it is the case that ${\displaystyle |P_{1}(V)|<|P(V)|<<|V|}$; Choice allows one not only to prove that there are urelements but that there are many cardinals between ${\displaystyle |P(V)|}$ and ${\displaystyle |V|}$.

One can now introduce some useful notions. A set ${\displaystyle A}$ which satisfies the intuitively appealing ${\displaystyle |A|=|P_{1}(A)|}$ is said to be Cantorian: a Cantorian set satisfies the usual form of Cantor's theorem. A set ${\displaystyle A}$ which satisfies the further condition that ${\displaystyle (x\mapsto \{x\})\lceil A}$, the restriction of the singleton map to A, is a set is not only Cantorian set but strongly Cantorian.

The Burali-Forti paradox of the largest ordinal number goes as follows. Define (following naive set theory) the ordinals as equivalence classes of well-orderings under isomorphism. There is an obvious natural well-ordering on the ordinals; since it is a well-ordering it belongs to an ordinal ${\displaystyle \Omega }$. It is straightforward to prove (by transfinite induction) that the order type of the natural order on the ordinals less than a given ordinal ${\displaystyle \alpha }$ is ${\displaystyle \alpha }$ itself. But this means that ${\displaystyle \Omega }$ is the order type of the ordinals ${\displaystyle <\Omega }$ and so is strictly less than the order type of all the ordinals — but the latter is, by definition, ${\displaystyle \Omega }$ itself!

The solution to the paradox in NF(U) starts with the observation that the order type of the natural order on the ordinals less than ${\displaystyle \alpha }$ is of a higher type than ${\displaystyle \alpha }$. Hence a type level ordered pair is two types higher than the type of its arguments and the usual Kuratowski ordered pair four types higher. For any order type ${\displaystyle \alpha }$, we can define an order type ${\displaystyle \alpha }$ one type higher: if ${\displaystyle W\in \alpha }$, then ${\displaystyle T(\alpha )}$ is the order type of the order ${\displaystyle W^{\iota }=\{(\{x\},\{y\})\mid xWy\}}$. The triviality of the T operation is only a seeming one; it is easy to show that T is a strictly monotone (order preserving) operation on the ordinals.

Now the lemma on order types may be restated in a stratified manner: the order type of the natural order on the ordinals ${\displaystyle <\alpha }$ is ${\displaystyle T^{2}(\alpha )}$ or ${\displaystyle T^{4}(\alpha )}$ depending on which pair is used (we assume the type level pair hereinafter). From this one may deduce that the order type on the ordinals ${\displaystyle <\Omega }$ is ${\displaystyle T^{2}(\Omega )}$, and thus ${\displaystyle T^{2}(\Omega )<\Omega }$. Hence the T operation is not a function; there cannot be a strictly monotone set map from ordinals to ordinals which sends an ordinal downward! Since T is monotone, we have ${\displaystyle \Omega >T^{2}(\Omega )>T^{4}(\Omega )\ldots }$, a "descending sequence" in the ordinals which cannot be a set.

One might assert that this result shows that no model of NF(U) is "standard", since the ordinals in any model of NFU are externally not well-ordered. One need not take a position on this, but can note that it is also a theorem of NFU that any set model of NFU has non-well-ordered "ordinals"; NFU does not conclude that the universe V is a model of NFU, despite V being a set, because the membership relation is not a set relation.

For a further development of mathematics in NFU, with a comparison to the development of the same in ZFC, see implementation of mathematics in set theory.

ML is an extension of NF that includes proper classes as well as sets. The set theory of the 1940 first edition of Quine's Mathematical Logic married NF to the proper classes of NBG set theory, and included an axiom schema of unrestricted comprehension for proper classes. However J. Barkley Rosser (1942) proved that the system presented in Mathematical Logic was subject to the Burali-Forti paradox. This result does not apply to NF. Hao Wang (1950) showed how to amend Quine's axioms for ML so as to avoid this problem, and Quine included the resulting axiomatization in the 1951 second and final edition of Mathematical Logic.

Wang proved that if NF is consistent then so is the revised ML, and also showed that the revised ML can prove the consistency of NF, that is that NF and the revised ML are equiconsistent.

There is a fairly simple method for producing models of NFU in bulk. Using well-known techniques of model theory, one can construct a nonstandard model of Zermelo set theory (nothing nearly as strong as full ZFC is needed for the basic technique) on which there is an external automorphism j (not a set of the model) which moves a rank ${\displaystyle V_{\alpha }}$ of the cumulative hierarchy of sets. We may suppose without loss of generality that ${\displaystyle j(\alpha )<\alpha }$. We talk about the automorphism moving the rank rather than the ordinal because we do not want to assume that every ordinal in the model is the index of a rank.

The domain of the model of NFU will be the nonstandard rank ${\displaystyle V_{\alpha }}$. The membership relation of the model of NFU will be

• ${\displaystyle x\in _{NFU}y\equiv _{def}j(x)\in y\wedge y\in V_{j(\alpha )+1}.}$

It may now be proved that this actually is a model of NFU. Let ${\displaystyle \phi }$ be a stratified formula in the language of NFU. Choose an assignment of types to all variables in the formula which witnesses the fact that it is stratified. Choose a natural number N greater than all types assigned to variables by this stratification.

Expand the formula ${\displaystyle \phi }$ into a formula ${\displaystyle \phi _{1}}$ in the language of the nonstandard model of Zermelo set theory with automorphism j using the definition of membership in the model of NFU. Application of any power of j to both sides of an equation or membership statement preserves its truth value because j is an automorphism. Make such an application to each atomic formula in ${\displaystyle \phi _{1}}$ in such a way that each variable x assigned type i occurs with exactly ${\displaystyle N-i}$ applications of j. This is possible thanks to the form of the atomic membership statements derived from NFU membership statements, and to the formula being stratified. Each quantified sentence ${\displaystyle (\forall x\in V_{\alpha }.\psi (j^{N-i}(x)))}$ can be converted to the form ${\displaystyle (\forall x\in j^{N-i}(V_{\alpha }).\psi (x))}$ (and similarly for existential quantifiers). Carry out this transformation everywhere and obtain a formula ${\displaystyle \phi _{2}}$ in which j is never applied to a bound variable.

Choose any free variable y in ${\displaystyle \phi }$ assigned type i. Apply ${\displaystyle j^{i-N}}$ uniformly to the entire formula to obtain a formula ${\displaystyle \phi _{3}}$ in which y appears without any application of j. Now ${\displaystyle \{y\in V_{\alpha }\mid \phi _{3}\}}$ exists (because j appears applied only to free variables and constants), belongs to ${\displaystyle V_{\alpha +1}}$, and contains exactly those y which satisfy the original formula ${\displaystyle \phi }$ in the model of NFU. ${\displaystyle j(\{y\in V_{\alpha }\mid \phi _{3}\})}$ has this extension in the model of NFU (the application of j corrects for the different definition of membership in the model of NFU). This establishes that Stratified Comprehension holds in the model of NFU.

To see that weak Extensionality holds is straightforward: each nonempty element of ${\displaystyle V_{j(\alpha )+1}}$ inherits a unique extension from the nonstandard model, the empty set inherits its usual extension as well, and all other objects are urelements.

The basic idea is that the automorphism j codes the "power set" ${\displaystyle V_{\alpha +1}}$ of our "universe" ${\displaystyle V_{\alpha }}$ into its externally isomorphic copy ${\displaystyle V_{j(\alpha )+1}}$ inside our "universe." The remaining objects not coding subsets of the universe are treated as urelements.

If ${\displaystyle \alpha }$ is a natural number n, one gets a model of NFU which claims that the universe is finite (it is externally infinite, of course). If ${\displaystyle \alpha }$ is infinite and the Choice holds in the nonstandard model of ZFC, one obtains a model of NFU + Infinity + Choice.

In this section, the effect is considered of adding various "strong axioms of infinity" to our usual base theory, NFU + Infinity + Choice. This base theory, known consistent, has the same strength as TST + Infinity, or Zermelo set theory with Separation restricted to bounded formulas (Mac Lane set theory).

One can add to this base theory strong axioms of infinity familiar from the ZFC context, such as "there exists an inaccessible cardinal," but it is more natural to consider assertions about Cantorian and strongly Cantorian sets. Such assertions not only bring into being large cardinals of the usual sorts, but strengthen the theory on its own terms.

The weakest of the usual strong principles is:

• Rosser's Axiom of Counting. The set of natural numbers is a strongly Cantorian set.

To see how natural numbers are defined in NFU, see set-theoretic definition of natural numbers. The original form of this axiom given by Rosser was "the set {m|1≤m≤n} has n members", for each natural number n. This intuitively obvious assertion is unstratified: what is provable in NFU is "the set {m|1≤m≤n} has ${\displaystyle T^{2}(n)}$ members" (where the T operation on cardinals is defined by ${\displaystyle T(|A|)=|P_{1}(A)|}$; this raises the type of a cardinal by one). For any cardinal number (including natural numbers) to assert ${\displaystyle T(|A|)=|A|}$ is equivalent to asserting that the sets A of that cardinality are Cantorian (by a usual abuse of language, we refer to such cardinals as "Cantorian cardinals"). It is straightforward to show that the assertion that each natural number is Cantorian is equivalent to the assertion that the set of all natural numbers is strongly Cantorian.

Counting is consistent with NFU, but increases its consistency strength noticeably; not, as one would expect, in the area of arithmetic, but in higher set theory. NFU + Infinity proves that each ${\displaystyle \beth _{n}}$ exists, but not that ${\displaystyle \beth _{\omega }}$ exists; NFU + Counting (easily) proves Infinity, and further proves the existence of ${\displaystyle \beth _{\beth _{n}}}$ for each n, but not the existence of ${\displaystyle \beth _{\beth _{\omega }}}$. (See beth numbers).

Counting implies immediately that one does not need to assign types to variables restricted to the set ${\displaystyle N}$ of natural numbers for purposes of stratification; it is a theorem that the power set of a strongly Cantorian set is strongly Cantorian, so it is further not necessary to assign types to variables restricted to any iterated power set of the natural numbers, or to such familiar sets as the set of real numbers, the set of functions from reals to reals, and so forth. The set-theoretical strength of Counting is less important in practice than the convenience of not having to annotate variables known to have natural number values (or related kinds of values) with singleton brackets, or to apply the T operation in order to get stratified set definitions.

Counting implies Infinity; each of the axioms below needs to be adjoined to NFU + Infinity to get the effect of strong variants of Infinity; Ali Enayat has investigated the strength of some of these axioms in models of NFU + "the universe is finite".

A model of the kind constructed above satisfies Counting just in case the automorphism j fixes all natural numbers in the underlying nonstandard model of Zermelo set theory.

The next strong axiom we consider is the

• Axiom of strongly Cantorian separation: For any strongly Cantorian set A and any formula ${\displaystyle \phi }$ (not necessarily stratified!) the set {x∈A|φ} exists.

Immediate consequences include Mathematical Induction for unstratified conditions (which is not a consequence of Counting; many but not all unstratified instances of induction on the natural numbers follow from Counting).

This axiom is surprisingly strong. Unpublished work of Robert Solovay shows that the consistency strength of the theory NFU* = NFU + Counting + Strongly Cantorian Separation is the same as that of Zermelo set theory + ${\displaystyle \Sigma _{2}}$ Replacement.

This axiom holds in a model of the kind constructed above (with Choice) if the ordinals which are fixed by j and dominate only ordinals fixed by j in the underlying nonstandard model of Zermelo set theory are standard, and the power set of any such ordinal in the model is also standard. This condition is sufficient but not necessary.

Next is

• Axiom of Cantorian Sets: Every Cantorian set is strongly Cantorian.

This very simple and appealing assertion is extremely strong. Solovay has shown the precise equivalence of the consistency strength of the theory NFUA = NFU + Infinity + Cantorian Sets with that of ZFC + a schema asserting the existence of an n-Mahlo cardinal for each concrete natural number n. Ali Enayat has shown that the theory of Cantorian equivalence classes of well-founded extensional relations (which gives a natural picture of an initial segment of the cumulative hierarchy of ZFC) interprets the extension of ZFC with n-Mahlo cardinals directly. A permutation technique can be applied to a model of this theory to give a model in which the hereditarily strongly Cantorian sets with the usual membership relation model the strong extension of ZFC.

This axiom holds in a model of the kind constructed above (with Choice) just in case the ordinals fixed by j in the underlying nonstandard model of ZFC are an initial (proper class) segment of the ordinals of the model.

Next consider the

• Axiom of Cantorian Separation: For any Cantorian set A and any formula ${\displaystyle \phi }$ (not necessarily stratified!) the set {x∈A|φ} exists.

This combines the effect of the two preceding axioms and is actually even stronger (precisely how is not known). Unstratified mathematical induction enables proving that there are n-Mahlo cardinals for every n, given Cantorian Sets, which gives an extension of ZFC that is even stronger than the previous one, which only asserts that there are n-Mahlos for each concrete natural number (leaving open the possibility of nonstandard counterexamples).

This axiom will hold in a model of the kind described above if every ordinal fixed by j is standard, and every power set of an ordinal fixed by j is also standard in the underlying model of ZFC. Again, this condition is sufficient but not necessary.

An ordinal is said to be Cantorian if it is fixed by T, and strongly Cantorian if it dominates only Cantorian ordinals (this implies that it is itself Cantorian). In models of the kind constructed above, Cantorian ordinals of NFU correspond to ordinals fixed by j (they are not the same objects because different definitions of ordinal numbers are used in the two theories).

Equal in strength to Cantorian Sets is the

• Axiom of Large Ordinals: For each non-Cantorian ordinal ${\displaystyle \alpha }$, there is a natural number n such that ${\displaystyle T^{n}(\Omega )<\alpha }$.

Recall that ${\displaystyle \Omega }$ is the order type of the natural order on all ordinals. This only implies Cantorian Sets if we have Choice (but is at that level of consistency strength in any case). It is remarkable that one can even define ${\displaystyle T^{n}(\Omega )}$: this is the nth term ${\displaystyle s_{n}}$ of any finite sequence of ordinals s of length n such that ${\displaystyle s_{0}=\Omega }$, ${\displaystyle s_{i+1}=T(s_{i})}$ for each appropriate i. This definition is completely unstratified. The uniqueness of ${\displaystyle T^{n}(\Omega )}$ can be proved (for those n for which it exists) and a certain amount of common-sense reasoning about this notion can be carried out, enough to show that Large Ordinals implies Cantorian Sets in the presence of Choice. In spite of the knotty formal statement of this axiom, it is a very natural assumption, amounting to making the action of T on the ordinals as simple as possible.

A model of the kind constructed above will satisfy Large Ordinals, if the ordinals moved by j are exactly the ordinals which dominate some ${\displaystyle j^{-i}(\alpha )}$ in the underlying nonstandard model of ZFC.

• Axiom of Small Ordinals: For any formula φ, there is a set A such that the elements of A which are strongly Cantorian ordinals are exactly the strongly Cantorian ordinals such that φ.

Solovay has shown the precise equivalence in consistency strength of NFUB = NFU + Infinity + Cantorian Sets + Small Ordinals with Morse–Kelley set theory plus the assertion that the proper class ordinal (the class of all ordinals) is a weakly compact cardinal. This is very strong indeed! Moreover, NFUB-, which is NFUB with Cantorian Sets omitted, is easily seen to have the same strength as NFUB.

A model of the kind constructed above will satisfy this axiom if every collection of ordinals fixed by j is the intersection of some set of ordinals with the ordinals fixed by j, in the underlying nonstandard model of ZFC.

Even stronger is the theory NFUM = NFU + Infinity + Large Ordinals + Small Ordinals. This is equivalent to Morse–Kelley set theory with a predicate on the classes which is a κ-complete nonprincipal ultrafilter on the proper class ordinal κ; in effect, this is Morse–Kelley set theory + "the proper class ordinal is a measurable cardinal"!

The technical details here are not the main point, which is that reasonable and natural (in the context of NFU) assertions turn out to be equivalent in power to very strong axioms of infinity in the ZFC context. This fact is related to the correlation between the existence of models of NFU, described above and satisfying these axioms, and the existence of models of ZFC with automorphisms having special properties.

• Alternative set theory
• Axiomatic set theory
• Implementation of mathematics in set theory
• Positive set theory
• Set-theoretic definition of natural numbers

• Crabbé, Marcel (1982). "On the consistency of an impredicative fragment of Quine's NF". The Journal of Symbolic Logic. 47 (1): 131–136. doi:10.2307/2273386. JSTOR 2273386.
• Forster, T. E. (1992), Set theory with a universal set. Exploring an untyped universe, Oxford Science Publications, Oxford Logic Guides, 20, New York: The Clarendon Press, Oxford University Press, ISBN 0-19-853395-0, MR 1166801
• Holmes, M. Randall (1998), Elementary set theory with a universal set (PDF), Cahiers du Centre de Logique, 10, Louvain-la-Neuve: Université Catholique de Louvain, Département de Philosophie, ISBN 2-87209-488-1, MR 1759289
• Jensen, R. B. (1969), "On the Consistency of a Slight(?) Modification of Quine's NF", Synthese, 19 (1/2): 250–63, doi:10.1007/bf00568059, JSTOR 20114640 With discussion by Quine.
• Quine, W. V. (1937), "New Foundations for Mathematical Logic", The American Mathematical Monthly, Mathematical Association of America, 44 (2): 70–80, doi:10.2307/2300564, JSTOR 2300564
• Quine, Willard Van Orman (1940), Mathematical Logic (first ed.), New York: W. W. Norton & Co., Inc., MR 0002508
• Quine, Willard Van Orman (1951), Mathematical logic (Revised ed.), Cambridge, Mass.: Harvard University Press, ISBN 0-674-55451-5, MR 0045661
• Quine, W. V., 1980, "New Foundations for Mathematical Logic" in From a Logical Point of View, 2nd ed., revised. Harvard Univ. Press: 80-101. The definitive version of where it all began, namely Quine's 1937 paper in the American Mathematical Monthly.
• Rosser, Barkley (1942), "The Burali-Forti paradox", Journal of Symbolic Logic, 7 (1): 1–17, doi:10.2307/2267550, JSTOR 2267550, MR 0006327
• Wang, Hao (1950), "A formal system of logic", Journal of Symbolic Logic, 15 (1): 25–32, doi:10.2307/2268438, JSTOR 2268438, MR 0034733
• Holmes, M. Randall (2008). "Symmetry as a Criterion for Comprehension Motivating Quine's 'New Foundations'". Studia Logica. 88 (2): 195–213. doi:10.1007/s11225-008-9107-8.

• "Enriched Stratified systems for the Foundations of Category Theory" by Solomon Feferman (2011)
• Stanford Encyclopedia of Philosophy:
  • Quine's New Foundations — by Thomas Forster.
  • Alternative axiomatic set theories — by Randall Holmes.
• Randall Holmes: New Foundations Home Page.
• Randall Holmes: Bibliography of Set Theory with a Universal Set.
• Randall Holmes: A new pass at the NF consistency proof