Consider $\{7,8,9\}$ and $\{7,8,10\}$ to be the informal representations of two sets. Formally, in naive first-order set theory, we define these sets as

$$\exists a\ \forall x\ (x \in a \iff (x=7 \lor x=8 \lor x=9))$$ $$\exists b\ \forall x\ (x \in b \iff (x=7 \lor x=8 \lor x=10))$$

How do we formally prove, using naive first-order set theory, that these sets are different?

Note that informally, I believe we could prove it thus:

From the Axiom of Comprehension and Axiom of Extensionality, we can prove that for each well-formed formula P(x) in FOL, there is a unique set of objects that satisfy P(x).

Let's assume this proof has already been given. Then since the objects satisfying $P_1(x)=x=7 \lor x=8 \lor x=9$ are different from the objects satisfying $P_2(x)=x=7 \lor x=8 \lor x=10$, the unique sets made of objects satisfying each of these formulas are different.

Here is the line of reasoning of my incomplete attempt at a formal proof, using first-order logic:

I take the two definitions of the sets as premises. I start with a subproof by existential elimination, with the premise:

  • Assume a is a set such that $\forall x\ (x \in a \iff (x=7 \lor x=8 \lor x=9))$.

Nested within, I start another subproof by existential elimination, with the premise:

  • Assume b is a set such that $\forall x\ (x \in v \iff (x=7 \lor x=8 \lor x=10))$.

The Axiom of Extensionality says

$$\forall a\ \forall b\ [\forall x\ (x \in a \iff x \in b) \implies a=b]$$

By Universal Elimination, I can state

$$\forall x\ (x \in a \iff x \in b) \implies a=b\tag{1}$$

At this point, I have a material conditional, and I want to prove $a \neq b$.

If I assume $a=b$, $(1)$ doesn't actually let me do anything with this assumption. In fact, using just the two Axioms (Comprehension and Extensionality) $a=b$ doesn't seem to lead to any other statement that I can infer.

After all, it seems to be the case that $\forall x\ (x \in a \iff x \in b)$ can be false and $a=b$ true, since we don't have a biconditional (another source of doubt for me in this material).

Edit: What do I mean by FOL?

I learned what I imagine is a very basic form of FOL which includes introduction and elimination inference rules for $\land,\lor,\implies,\iff,\lnot$, quantifiers $\forall, \exists$, introduction and elimination inference rules for $\forall$ and $\exists$, plus the identity predicate.

When naive set theory was introduced to me, there was only one symbol, $\in$, the Axiom of Comprehension, and the Axiom of Extensionality.

Axiom of Comprehension

$$\exists a \forall [x \in a \iff P(x)]$$

Axiom of Extensionality

$$\forall a \forall b [\forall x (x \in a \iff x \in b) \implies a = b]$$

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    $\begingroup$ Formally speaking, in the most annoying sense of the word, the symbols $7,8,9,10$ are not part of the language of set theory. So unless you provide explicit definitions of them, we cannot determine if the two sets are equal or not. $\endgroup$
    – Asaf Karagila
    Commented Jan 30, 2022 at 12:31
  • $\begingroup$ Use Substitution axiom for equality: $A=B → (x∈A ↔ x∈B)$. Assuming $A=B$ we deduce that $∀x(x ∈ \{ 7,8,9 \} ↔ x ∈ \{ 7,8,10 \})$. From it, by Universal Instantiation, we derive: $9 ∈ \{ 7,8,9 \} ↔ 9 ∈ \{ 7,8,10 \}$ which is False. Having deduced a contradiction, we apply Negation Introduction to conclude with: $\lnot (A = B)$. $\endgroup$ Commented Feb 1, 2022 at 11:42
  • $\begingroup$ The needed instance of the equality axiom is: $A=B \to ((x \in z)[A/z] \leftrightarrow (x \in z)[B/z])$ $\endgroup$ Commented Feb 1, 2022 at 13:44

3 Answers 3


You don't need extensionality to prove that $\{ 7, 8, 9 \} \neq \{ 7, 8, 10 \}$.$^*$ Simply observe that $9 \in \{7, 8, 9 \}$ but $9 \notin \{7, 8, 10 \}$, therefore $\{ 7, 8, 9 \} \neq \{ 7, 8, 10 \}$.

If you want to write this last step in more detail, you could say that if $9 \in \{ 7, 8, 9 \}$ and $\{ 7, 8, 9 \} = \{ 7, 8, 10 \}$, then $9 \in \{ 7, 8, 10 \}$, but this is false, hence $\{ 7, 8, 9 \} \neq \{ 7, 8, 10 \}$.

Edit: since you ask for a formal proof:

  1. $\forall x (x \in \{ 7, 8, 9 \} \leftrightarrow (x = 7 \vee x = 8 \vee x = 9))$.
  2. $\forall x (x \in \{ 7, 8, 10 \} \leftrightarrow (x = 7 \vee x = 8 \vee x = 10))$.
  3. $9 \in \{ 7, 8, 9 \}$. (By universal instantiation. Take $x = 9$ in 1.)
  4. $9 \notin \{ 7, 8, 10 \}$. (By universal instantiation. Take $x = 9$ in 2.)
  5. $\{ 7, 8, 9 \} = \{ 7, 8, 10 \} \rightarrow (9 \in \{ 7, 8, 9 \} \rightarrow 9 \in \{ 7, 8, 10 \})$. (This, or some equivalent formula, is an instance of an axiom of predicate logic with equality.)
  6. $\{ 7, 8, 9 \} \neq \{ 7, 8, 10 \}$. (By propositional logic.)

$^*$ Although if extensionality did not hold, then one could not make much sense of the notation $\{ 7, 8, 9 \}$ in the first place, since $\{ 7, 8, 9 \}$ is defined to be the unique set (unique by extensionality!) which contains the elements $7, 8, 9$ and nothing else.

  • 1
    $\begingroup$ @Hermis14 The axiom of extensionality states that if two sets have the same elements, then they are equal. That two sets cannot be equal if one contains an element that the other one does not is simply a basic principle of first-order logic, namely that identical objects have the same properties. (This is the same principle at play which allows us to infer e.g. that $m \neq n$ if $m$ is even but $n$ is not even.) That $\exists x (x \in y \wedge x \notin z) \rightarrow (y \neq z)$ is true is simply a matter of first-order logic, independent of any axioms of set theory. $\endgroup$
    – Pilcrow
    Commented Jan 30, 2022 at 12:54
  • 1
    $\begingroup$ @Hermis14 No, the principle $x = y \rightarrow \forall z (z \in x \leftrightarrow z \in y)$ is valid in first-order logic. $\endgroup$
    – Pilcrow
    Commented Jan 30, 2022 at 13:00
  • 1
    $\begingroup$ @Pilcrow I think that makes sense... Thank you for correcting me :) $\endgroup$
    – Hermis14
    Commented Jan 30, 2022 at 13:02
  • 1
    $\begingroup$ @evianpring I agree, which is why I said that the axiom of extensionality is irrelevant when you wish to show that the two sets are distinct. What is used it the principle that identical objects satisfy the same formulas. $\endgroup$
    – Pilcrow
    Commented Jan 30, 2022 at 15:54
  • 1
    $\begingroup$ @evianpring en.wikipedia.org/wiki/First-order_logic#Equality_and_its_axioms $\endgroup$
    – Pilcrow
    Commented Feb 1, 2022 at 11:39

We want to prove that $\{7,8,9\}=\{7,8,10\}$ is a false stetement.

Let's assume the premise is true.

We have $\forall x(x\in \{7,8,9\} \iff x\in \{7,8,10\})$, by definition.

This implies that $9\in \{7,8,9\} \iff 9\in \{7,8,10\}$. This is where we use the universal instantation which you mentioned in your post. Since the statement is true for ANY value of x, that means that substituting x with ANYTHING will still gice us a true statement. In this example, we have substitued x with 9.

$9=7 \lor 9=8 \lor 9=9 \iff 9=7 \lor 9=8 \lor 9=10$, using your own definition of an element being part of a set.

$false \lor false \lor true \iff false \lor false \lor false$

$true \iff false$


The premise leads to an impossible conclusion. From this we can conclude that the sets are indeed not equal.


First, let's investigate Axiom of Extensionality.

Axiom of Extensionality. For any two sets $A,B$,

\begin{equation} (\text{for any }x\text{ we have }x\in A\text{ if and only if }x\in B)\Leftrightarrow A=B. \end{equation}

In fact, Axiom of Extensionality is $\Rightarrow$ while $\Leftarrow$ follows the meaning of $=$. "$\Rightarrow$" is equivalent of say "If the extensions of $A$ and $B$ are the same then $A$ and $B$ are equal". "$\Leftarrow$" is equivalent to say "If $A$ and $B$ are equal then the extensions of $A$ and $B$ are the same" which is a truth in all the truth theory which is determined since if $A$ and $B$ are equal then everything of $A$ and $B$ are the same. Although of this, we usually combine the two arrows together since "$\Leftarrow$" is obvious.

Now turn to your problem. To show $\{7,8,9\}\neq\{8,9,10\}$ is easy that we just need to use "$\Leftarrow$" which is a truth while in fact not Axiom of Extentionality.

Proof 1. Suppose $\{7,8,9\}=\{8,9,10\}$. Since $7\in\{7,8,9\}$, then by $\{7,8,9\}=\{8,9,10\}$ we have $7\in\{8,9,10\}$, clearly contradicted to $7\notin\{8,9,10\}$.

Proof 2. Suppose $\{7,8,9\}=\{8,9,10\}$. Since $10\in\{8,9,10\}$, then by $\{7,8,9\}=\{8,9,10\}$ we have $10\in\{7,8,9\}$, clearly contradicted to $10\notin\{7,8,9\}$.


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