# Does Regularity schema imply $\in$-induction when added to first order Zermelo set theory?

That $$\in$$-induction fails to be a theorem schema of first order Zermelo + Foundation (see here), then it appears that it is more eligible to replace axiom of Regularity (Foundation) by a Regularity schema:

Axiom Schema of Regularity: if $$\varphi(x,y_1,...,y_n)$$ is a formula in which only symbols $$x,y_1,..,y_n"$$ occur free (and only free), and in which the symbol $$z"$$ doesn't occur, and $$\varphi(z,y_1,..,y_n)$$ is the formula obtained from formula $$\varphi(x,y_1,..,y_n)$$ by merely replacing each occurrence of the symbol $$x"$$ in it by the symbol $$z"$$, then

$$\forall y_1,..,y_n [\exists x (\varphi(x,y_1,..,y_n)) \to \exists x (\varphi(x,y_1,..,y_n) \wedge \not \exists z \in x (\varphi(z,y_1,..,y_n)))]$$

is an axiom.

In short this says: $$\exists \varphi(x) \to \neg \forall \varphi(x) \exists \varphi(y) \in x$$ (parameters allowed)

It is clear that this is a schematic rendering of axiom of foundation, by re-presenting it in terms of predicates instead of sets.

Now I think that first order Zermelo + Regularity schema would imply $$\in$$-induction.

Is that correct?

Yes; the axiom schemas of regularity and $$\in$$-induction are equivalent, in fact by first-order logic alone.

The argument is clearest presented in the language of classes, in the style of NBG set theory. To make this an argument in $$Z$$, just replace each mention of a class $$C$$ with a formula $$\varphi(x,y_1,\ldots,y_n)$$, viewed as a predicate on $$x$$ with parameters $$y_1,\ldots,y_n$$.

In terms of classes, the instance of $$\in$$-induction for a given class $$C$$ says “If $$C$$ is $$\in$$-hereditary, then $$C$$ contains all sets.” Similarly, the instance of regularity for a given class $$D$$ says: “if $$D$$ is inhabited, then $$D$$ has an $$\in$$-minimal element.”

But “$$\in$$-hereditary” is dual to “has an $$\in$$-minimal element”, in the sense that a class $$C$$ is hereditary precisely if its complement $$\bar{C}$$ does not have an $$\in$$-minimal element, and “contains all sets” is dual to “is inhabited” in the same sense.

So we have a chain of equivalent statements, using purely first-order logic:

• If $$C$$ is $$\in$$-hereditary, then $$C$$ contains all sets
• (contrapositive) If $$C$$ does not contain all sets, then $$C$$ is not $$\in$$-hereditary.
• (duality as noted above) If $$\bar{C}$$ is inhabited, then $$\bar{C}$$ has an $$\in$$-minimal element.

So this shows: the instance of $$\in$$-induction for any class $$C$$ is equivalent to the regularity for its complement $$\bar{C}$$, and vice versa.

Going back to the first-order axiom-scheme versions, we get that an arbitrary instance of $$\in$$-induction, for some formula $$\varphi(x,y_1,\ldots,y_n)$$, is equivalent to the instance of regularity for the complementary formula $$\lnot \varphi(x,y_1,\ldots,y_n)$$; and similarly, every instance of regularity for a formula $$\varphi$$ is equivalent to the instance of $$\in$$-induction for $$\lnot \varphi$$.

• The first line of your statement is not careful, since the known regularity axiom in the context of Z is not over classes, it is over $sets$, and in this sense it is not equivalent to $\in$-induction as shown in the referred link. So you need to be careful, I think you mean "Regularity OVER CLASSES" is equivalent to "$\in$-induction over CLASSES", and indeed the first does imply the Regularity schema I'm speaking about. However I'm not sure if the regularity scheme I'm speaking implies your Regularity over classes, which appears to me to be stronger since it cover indefinable classes as well. Nov 8, 2018 at 17:19
• Sorry if my writing in terms of classes was distracting; but the argument applies directly to the usual schemas for regularity/$\in$-induction in terms of formulas/definable classes, as I say in my final summary paragraph. To write the whole argument out in terms of formulas, just replace the class $C$ throughout by a formula $\varphi(x,y_1,\ldots,y_n)$, where $y_1,\ldots,y_n$ are considered as parameters, and replace “$C$ contains all sets” with “$\varphi(x,y_1,\ldots,y_n)$ holds for all sets $x$”, and so on. Nov 8, 2018 at 21:08
• By the way, this style of arguing — speaking of “classes” and often also “class functions” rather than explicitly of formulas defining them — is quite common in set theory, and worth getting used to. It’s entirely valid, and completely direct to translate into elementary first-order arguments, so long as there’s no use of quantification over “arbitrary classes” or “arbitrary class functions” apart from implicit universal quantification at the outermost level. Nov 8, 2018 at 21:34
• still for precision purposes, you need to change the first line of your answer to Regularity [over classes] and $\in$-induction are equivalent. Otherwise it could be misunderstood, since in the milieu of Z Regularity is not a schema, it is just a single axiom applied over sets only. Nov 9, 2018 at 11:29
• No the standard version for Regularity in set theory is a single axiom: $\forall x [\exists y (y \in x) \to \exists y \in x (\neg \exists c( c \in y \wedge c \in x))]$, this is a single axiom it is not a schema in the sense that it has many instances subtituting a symbol of the meta-theoretic langauge that range over formulas of the language. While in this post Regularity is not presented in this way, it is presented as a schema. Actually Zermelo didn't present Regularity among his original axioms, and when one say Z+Regularity, it is meant the Regularity axiom of ZF which is not a schema! Nov 9, 2018 at 14:35

I'll re-present the answer of Lumsdaine that both schemes are equivalent in Classical first order logic, but in a more direct manner.

I'll start with $$\in$$-induction to end up in Regularity scheme, using only Classical First order logic with membership.

$$\forall x [\forall y \in x (\varphi(y)) \to \varphi(x)] \to \forall x (\varphi(x))$$, we negate both sides to get:

$$\neg\forall x (\varphi(x)) \to \neg \forall x [\forall y \in x (\varphi(y)) \to \varphi(x)]$$

$$\exists x (\neg \varphi(x)) \to \exists x [\forall y \in x (\varphi(y)) \wedge \neg \varphi(x)]$$

Let $$\neg\varphi \iff \psi$$, then by rule of excluded middle: $$\varphi \iff \neg \psi$$, then:

$$\exists x (\psi(x)) \to \exists x [\forall y \in x (\neg\psi(y)) \wedge \psi(x)]$$

$$\exists x (\psi(x)) \to \exists x [\psi(x)\wedge\not \exists y \in x (\psi(y)) ]$$

Which is the regularity schema.

So both $$\in$$-induction and Regularity schema are reformulations of each other in classical first order logic with membership $$\in$$.

Also there is another way, albeit indirect, to show this equivalence.

Lets say that a predicate $$\varphi$$ is ascending if when it is fulfilled by all members of a set then it is fulfilled by that set itself (i.e. it meets the antecedent of $$\in$$-induction). On the other hand lets say that a predicate $$\varphi$$ is descending if when it is fulfilled by a set then there must be an element of that set that fulfills it. Now a property is said to be universal if it is fulfilled by all sets, and inhabited if it is fulfilled by at least one set.

1. Regularity scheme say that all inhabited predicates are non-descending.
2. $$\in$$-induction scheme says that all ascending predicates are universal.

Now if we assume 1. then clearly any counter-example to 2. would be a descending predicate thus violating the consequent of 1.! So 1. implies 2.

Now if we assume 2. then suppose that $$\varphi$$ is inhabited and descending, then $$\neg \varphi$$ is ascending, and thus $$\varphi$$ would violate the consequent of $$\in$$-induction for predicate $$\neg \varphi$$. So 2. implies 1.

Thus $$1. \iff 2.$$

However this indirect argument of equivalence also requires the law of excluded middle, and so it is only valid in classical first order logic.

• "In a more direct manner" depends on points of view. E. g. for me, another answer is MUCH easier to grasp. Nov 9, 2018 at 21:27