Why can't we just add “nothing else is a set” as an axiom?












7












$begingroup$


The axioms of ZF define what a set is by:





  1. $omega$ is a set

  2. If $x$ and $y$ are sets, then ${x, y}$ is a set

  3. If $x$ is a set, then $bigcup x$ is a set

  4. If $x$ is a set, then $mathcal{P}(x)$ is a set

  5. If $x, y_1, ..., y_n$ are sets and $P$ is a statement free variables $x, y, z_1, ..., z_n$, then ${z in x : P}$ is a set


and also defines equality between sets by the axiom of extensionality.



But I don't understand why we can't just add "nothing else is a set" as an axiom to the list, as we usually do when defining a certain mathematical object.



Wouldn't adding that axiom solve the problem of some statements being independent? If we can't prove a set exists, then it means it can't be produced using the rules listed above, so it's not a set.










share|cite|improve this question









$endgroup$








  • 3




    $begingroup$
    You forgot collection/replacement
    $endgroup$
    – Not Mike
    Jan 18 at 20:36






  • 1




    $begingroup$
    For example, there is a set called $mathcal P(omega)$, but it has only countably many elements (only countably many can be proved from these axioms, and "nothing else is a set").
    $endgroup$
    – GEdgar
    Jan 18 at 20:40






  • 1




    $begingroup$
    See also en.wikipedia.org/wiki/Constructible_universe
    $endgroup$
    – Not Mike
    Jan 18 at 20:55








  • 5




    $begingroup$
    "The axioms of ZF define what a set is..." Where does this misconception come from? I've seen it so many times...
    $endgroup$
    – Jonathan
    Jan 18 at 22:30






  • 1




    $begingroup$
    @Jonathan: Yes, but if you describe the structure of a vector space, then you could say that you're talking about properties of vectors (e.g., closed under addition and scalar multiplication). So in that sense, why wouldn't it be correct to say that the axioms of ZF describe the properties of sets (e.g., if $x$ is a set, then it has a power set, and subclasses of $x$ are sets, etc.)?
    $endgroup$
    – Asaf Karagila
    Jan 20 at 0:45
















7












$begingroup$


The axioms of ZF define what a set is by:





  1. $omega$ is a set

  2. If $x$ and $y$ are sets, then ${x, y}$ is a set

  3. If $x$ is a set, then $bigcup x$ is a set

  4. If $x$ is a set, then $mathcal{P}(x)$ is a set

  5. If $x, y_1, ..., y_n$ are sets and $P$ is a statement free variables $x, y, z_1, ..., z_n$, then ${z in x : P}$ is a set


and also defines equality between sets by the axiom of extensionality.



But I don't understand why we can't just add "nothing else is a set" as an axiom to the list, as we usually do when defining a certain mathematical object.



Wouldn't adding that axiom solve the problem of some statements being independent? If we can't prove a set exists, then it means it can't be produced using the rules listed above, so it's not a set.










share|cite|improve this question









$endgroup$








  • 3




    $begingroup$
    You forgot collection/replacement
    $endgroup$
    – Not Mike
    Jan 18 at 20:36






  • 1




    $begingroup$
    For example, there is a set called $mathcal P(omega)$, but it has only countably many elements (only countably many can be proved from these axioms, and "nothing else is a set").
    $endgroup$
    – GEdgar
    Jan 18 at 20:40






  • 1




    $begingroup$
    See also en.wikipedia.org/wiki/Constructible_universe
    $endgroup$
    – Not Mike
    Jan 18 at 20:55








  • 5




    $begingroup$
    "The axioms of ZF define what a set is..." Where does this misconception come from? I've seen it so many times...
    $endgroup$
    – Jonathan
    Jan 18 at 22:30






  • 1




    $begingroup$
    @Jonathan: Yes, but if you describe the structure of a vector space, then you could say that you're talking about properties of vectors (e.g., closed under addition and scalar multiplication). So in that sense, why wouldn't it be correct to say that the axioms of ZF describe the properties of sets (e.g., if $x$ is a set, then it has a power set, and subclasses of $x$ are sets, etc.)?
    $endgroup$
    – Asaf Karagila
    Jan 20 at 0:45














7












7








7





$begingroup$


The axioms of ZF define what a set is by:





  1. $omega$ is a set

  2. If $x$ and $y$ are sets, then ${x, y}$ is a set

  3. If $x$ is a set, then $bigcup x$ is a set

  4. If $x$ is a set, then $mathcal{P}(x)$ is a set

  5. If $x, y_1, ..., y_n$ are sets and $P$ is a statement free variables $x, y, z_1, ..., z_n$, then ${z in x : P}$ is a set


and also defines equality between sets by the axiom of extensionality.



But I don't understand why we can't just add "nothing else is a set" as an axiom to the list, as we usually do when defining a certain mathematical object.



Wouldn't adding that axiom solve the problem of some statements being independent? If we can't prove a set exists, then it means it can't be produced using the rules listed above, so it's not a set.










share|cite|improve this question









$endgroup$




The axioms of ZF define what a set is by:





  1. $omega$ is a set

  2. If $x$ and $y$ are sets, then ${x, y}$ is a set

  3. If $x$ is a set, then $bigcup x$ is a set

  4. If $x$ is a set, then $mathcal{P}(x)$ is a set

  5. If $x, y_1, ..., y_n$ are sets and $P$ is a statement free variables $x, y, z_1, ..., z_n$, then ${z in x : P}$ is a set


and also defines equality between sets by the axiom of extensionality.



But I don't understand why we can't just add "nothing else is a set" as an axiom to the list, as we usually do when defining a certain mathematical object.



Wouldn't adding that axiom solve the problem of some statements being independent? If we can't prove a set exists, then it means it can't be produced using the rules listed above, so it's not a set.







set-theory






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Jan 18 at 19:52









StefanStefan

1906




1906








  • 3




    $begingroup$
    You forgot collection/replacement
    $endgroup$
    – Not Mike
    Jan 18 at 20:36






  • 1




    $begingroup$
    For example, there is a set called $mathcal P(omega)$, but it has only countably many elements (only countably many can be proved from these axioms, and "nothing else is a set").
    $endgroup$
    – GEdgar
    Jan 18 at 20:40






  • 1




    $begingroup$
    See also en.wikipedia.org/wiki/Constructible_universe
    $endgroup$
    – Not Mike
    Jan 18 at 20:55








  • 5




    $begingroup$
    "The axioms of ZF define what a set is..." Where does this misconception come from? I've seen it so many times...
    $endgroup$
    – Jonathan
    Jan 18 at 22:30






  • 1




    $begingroup$
    @Jonathan: Yes, but if you describe the structure of a vector space, then you could say that you're talking about properties of vectors (e.g., closed under addition and scalar multiplication). So in that sense, why wouldn't it be correct to say that the axioms of ZF describe the properties of sets (e.g., if $x$ is a set, then it has a power set, and subclasses of $x$ are sets, etc.)?
    $endgroup$
    – Asaf Karagila
    Jan 20 at 0:45














  • 3




    $begingroup$
    You forgot collection/replacement
    $endgroup$
    – Not Mike
    Jan 18 at 20:36






  • 1




    $begingroup$
    For example, there is a set called $mathcal P(omega)$, but it has only countably many elements (only countably many can be proved from these axioms, and "nothing else is a set").
    $endgroup$
    – GEdgar
    Jan 18 at 20:40






  • 1




    $begingroup$
    See also en.wikipedia.org/wiki/Constructible_universe
    $endgroup$
    – Not Mike
    Jan 18 at 20:55








  • 5




    $begingroup$
    "The axioms of ZF define what a set is..." Where does this misconception come from? I've seen it so many times...
    $endgroup$
    – Jonathan
    Jan 18 at 22:30






  • 1




    $begingroup$
    @Jonathan: Yes, but if you describe the structure of a vector space, then you could say that you're talking about properties of vectors (e.g., closed under addition and scalar multiplication). So in that sense, why wouldn't it be correct to say that the axioms of ZF describe the properties of sets (e.g., if $x$ is a set, then it has a power set, and subclasses of $x$ are sets, etc.)?
    $endgroup$
    – Asaf Karagila
    Jan 20 at 0:45








3




3




$begingroup$
You forgot collection/replacement
$endgroup$
– Not Mike
Jan 18 at 20:36




$begingroup$
You forgot collection/replacement
$endgroup$
– Not Mike
Jan 18 at 20:36




1




1




$begingroup$
For example, there is a set called $mathcal P(omega)$, but it has only countably many elements (only countably many can be proved from these axioms, and "nothing else is a set").
$endgroup$
– GEdgar
Jan 18 at 20:40




$begingroup$
For example, there is a set called $mathcal P(omega)$, but it has only countably many elements (only countably many can be proved from these axioms, and "nothing else is a set").
$endgroup$
– GEdgar
Jan 18 at 20:40




1




1




$begingroup$
See also en.wikipedia.org/wiki/Constructible_universe
$endgroup$
– Not Mike
Jan 18 at 20:55






$begingroup$
See also en.wikipedia.org/wiki/Constructible_universe
$endgroup$
– Not Mike
Jan 18 at 20:55






5




5




$begingroup$
"The axioms of ZF define what a set is..." Where does this misconception come from? I've seen it so many times...
$endgroup$
– Jonathan
Jan 18 at 22:30




$begingroup$
"The axioms of ZF define what a set is..." Where does this misconception come from? I've seen it so many times...
$endgroup$
– Jonathan
Jan 18 at 22:30




1




1




$begingroup$
@Jonathan: Yes, but if you describe the structure of a vector space, then you could say that you're talking about properties of vectors (e.g., closed under addition and scalar multiplication). So in that sense, why wouldn't it be correct to say that the axioms of ZF describe the properties of sets (e.g., if $x$ is a set, then it has a power set, and subclasses of $x$ are sets, etc.)?
$endgroup$
– Asaf Karagila
Jan 20 at 0:45




$begingroup$
@Jonathan: Yes, but if you describe the structure of a vector space, then you could say that you're talking about properties of vectors (e.g., closed under addition and scalar multiplication). So in that sense, why wouldn't it be correct to say that the axioms of ZF describe the properties of sets (e.g., if $x$ is a set, then it has a power set, and subclasses of $x$ are sets, etc.)?
$endgroup$
– Asaf Karagila
Jan 20 at 0:45










2 Answers
2






active

oldest

votes


















11












$begingroup$

It's not clear why we need to. One reason we "can't" has to do with a technicality. ZF is a theory in "first-order logic". Saying that nothing is a set except that which the axioms imply is a set is not a "first-order" statement, so if we tried to add it we'd no longer have a first-order theory. That would be too bad, because first-order logic works out a lot nicer than higher-order logics.



There's no way to say "nothing is a set except things that the axioms imply are sets" using just $forall$, $exists$ and $in$.



(In fact, the axioms don't quite say what you say they say! There's no mention of "is a set" in the actual axioms. What you say is maybe how one thinks off what the axioms mean, but in fact, for example the actual axiom (2) is $$forall xforall yexists z(forall t(tin ziff (t= xlor t= y))).$$The axioms talk about sets, not about what is or is not a set; adding "is a set" to the formalism makes it a totally different sort of thing.)






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
    $endgroup$
    – Arturo Magidin
    Jan 18 at 20:57






  • 1




    $begingroup$
    @ArturoMagidin Yes of course, wasn't thinking.
    $endgroup$
    – David C. Ullrich
    Jan 18 at 21:00



















1












$begingroup$

Your proposed rule would lead to contradictions. Take the continuum hypothesis, for example. We can't prove (in ZFC, assuming ZFC is consistent) that there is a set of cardinality between that of $mathbb{N}$ and $mathbb{R}$, so the rule says there isn't one. So, given the rule, the continuum hypothesis is true.



But wait! We also can't prove that there is a bijection between $omega_1$ and $mathbb{R}$, so the rule says there isn't one, and the continuum hypothesis is false.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
    $endgroup$
    – Andrés E. Caicedo
    Jan 19 at 12:59











Your Answer





StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3078680%2fwhy-cant-we-just-add-nothing-else-is-a-set-as-an-axiom%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























2 Answers
2






active

oldest

votes








2 Answers
2






active

oldest

votes









active

oldest

votes






active

oldest

votes









11












$begingroup$

It's not clear why we need to. One reason we "can't" has to do with a technicality. ZF is a theory in "first-order logic". Saying that nothing is a set except that which the axioms imply is a set is not a "first-order" statement, so if we tried to add it we'd no longer have a first-order theory. That would be too bad, because first-order logic works out a lot nicer than higher-order logics.



There's no way to say "nothing is a set except things that the axioms imply are sets" using just $forall$, $exists$ and $in$.



(In fact, the axioms don't quite say what you say they say! There's no mention of "is a set" in the actual axioms. What you say is maybe how one thinks off what the axioms mean, but in fact, for example the actual axiom (2) is $$forall xforall yexists z(forall t(tin ziff (t= xlor t= y))).$$The axioms talk about sets, not about what is or is not a set; adding "is a set" to the formalism makes it a totally different sort of thing.)






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
    $endgroup$
    – Arturo Magidin
    Jan 18 at 20:57






  • 1




    $begingroup$
    @ArturoMagidin Yes of course, wasn't thinking.
    $endgroup$
    – David C. Ullrich
    Jan 18 at 21:00
















11












$begingroup$

It's not clear why we need to. One reason we "can't" has to do with a technicality. ZF is a theory in "first-order logic". Saying that nothing is a set except that which the axioms imply is a set is not a "first-order" statement, so if we tried to add it we'd no longer have a first-order theory. That would be too bad, because first-order logic works out a lot nicer than higher-order logics.



There's no way to say "nothing is a set except things that the axioms imply are sets" using just $forall$, $exists$ and $in$.



(In fact, the axioms don't quite say what you say they say! There's no mention of "is a set" in the actual axioms. What you say is maybe how one thinks off what the axioms mean, but in fact, for example the actual axiom (2) is $$forall xforall yexists z(forall t(tin ziff (t= xlor t= y))).$$The axioms talk about sets, not about what is or is not a set; adding "is a set" to the formalism makes it a totally different sort of thing.)






share|cite|improve this answer











$endgroup$













  • $begingroup$
    Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
    $endgroup$
    – Arturo Magidin
    Jan 18 at 20:57






  • 1




    $begingroup$
    @ArturoMagidin Yes of course, wasn't thinking.
    $endgroup$
    – David C. Ullrich
    Jan 18 at 21:00














11












11








11





$begingroup$

It's not clear why we need to. One reason we "can't" has to do with a technicality. ZF is a theory in "first-order logic". Saying that nothing is a set except that which the axioms imply is a set is not a "first-order" statement, so if we tried to add it we'd no longer have a first-order theory. That would be too bad, because first-order logic works out a lot nicer than higher-order logics.



There's no way to say "nothing is a set except things that the axioms imply are sets" using just $forall$, $exists$ and $in$.



(In fact, the axioms don't quite say what you say they say! There's no mention of "is a set" in the actual axioms. What you say is maybe how one thinks off what the axioms mean, but in fact, for example the actual axiom (2) is $$forall xforall yexists z(forall t(tin ziff (t= xlor t= y))).$$The axioms talk about sets, not about what is or is not a set; adding "is a set" to the formalism makes it a totally different sort of thing.)






share|cite|improve this answer











$endgroup$



It's not clear why we need to. One reason we "can't" has to do with a technicality. ZF is a theory in "first-order logic". Saying that nothing is a set except that which the axioms imply is a set is not a "first-order" statement, so if we tried to add it we'd no longer have a first-order theory. That would be too bad, because first-order logic works out a lot nicer than higher-order logics.



There's no way to say "nothing is a set except things that the axioms imply are sets" using just $forall$, $exists$ and $in$.



(In fact, the axioms don't quite say what you say they say! There's no mention of "is a set" in the actual axioms. What you say is maybe how one thinks off what the axioms mean, but in fact, for example the actual axiom (2) is $$forall xforall yexists z(forall t(tin ziff (t= xlor t= y))).$$The axioms talk about sets, not about what is or is not a set; adding "is a set" to the formalism makes it a totally different sort of thing.)







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Jan 18 at 20:59

























answered Jan 18 at 20:37









David C. UllrichDavid C. Ullrich

60.9k43994




60.9k43994












  • $begingroup$
    Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
    $endgroup$
    – Arturo Magidin
    Jan 18 at 20:57






  • 1




    $begingroup$
    @ArturoMagidin Yes of course, wasn't thinking.
    $endgroup$
    – David C. Ullrich
    Jan 18 at 21:00


















  • $begingroup$
    Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
    $endgroup$
    – Arturo Magidin
    Jan 18 at 20:57






  • 1




    $begingroup$
    @ArturoMagidin Yes of course, wasn't thinking.
    $endgroup$
    – David C. Ullrich
    Jan 18 at 21:00
















$begingroup$
Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
$endgroup$
– Arturo Magidin
Jan 18 at 20:57




$begingroup$
Are you sure the Axiom of pairs is what you write? Shouldn’t that be $t=x lor t=y$? Otherwise, what you get is $xcup y$.
$endgroup$
– Arturo Magidin
Jan 18 at 20:57




1




1




$begingroup$
@ArturoMagidin Yes of course, wasn't thinking.
$endgroup$
– David C. Ullrich
Jan 18 at 21:00




$begingroup$
@ArturoMagidin Yes of course, wasn't thinking.
$endgroup$
– David C. Ullrich
Jan 18 at 21:00











1












$begingroup$

Your proposed rule would lead to contradictions. Take the continuum hypothesis, for example. We can't prove (in ZFC, assuming ZFC is consistent) that there is a set of cardinality between that of $mathbb{N}$ and $mathbb{R}$, so the rule says there isn't one. So, given the rule, the continuum hypothesis is true.



But wait! We also can't prove that there is a bijection between $omega_1$ and $mathbb{R}$, so the rule says there isn't one, and the continuum hypothesis is false.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
    $endgroup$
    – Andrés E. Caicedo
    Jan 19 at 12:59
















1












$begingroup$

Your proposed rule would lead to contradictions. Take the continuum hypothesis, for example. We can't prove (in ZFC, assuming ZFC is consistent) that there is a set of cardinality between that of $mathbb{N}$ and $mathbb{R}$, so the rule says there isn't one. So, given the rule, the continuum hypothesis is true.



But wait! We also can't prove that there is a bijection between $omega_1$ and $mathbb{R}$, so the rule says there isn't one, and the continuum hypothesis is false.






share|cite|improve this answer









$endgroup$













  • $begingroup$
    No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
    $endgroup$
    – Andrés E. Caicedo
    Jan 19 at 12:59














1












1








1





$begingroup$

Your proposed rule would lead to contradictions. Take the continuum hypothesis, for example. We can't prove (in ZFC, assuming ZFC is consistent) that there is a set of cardinality between that of $mathbb{N}$ and $mathbb{R}$, so the rule says there isn't one. So, given the rule, the continuum hypothesis is true.



But wait! We also can't prove that there is a bijection between $omega_1$ and $mathbb{R}$, so the rule says there isn't one, and the continuum hypothesis is false.






share|cite|improve this answer









$endgroup$



Your proposed rule would lead to contradictions. Take the continuum hypothesis, for example. We can't prove (in ZFC, assuming ZFC is consistent) that there is a set of cardinality between that of $mathbb{N}$ and $mathbb{R}$, so the rule says there isn't one. So, given the rule, the continuum hypothesis is true.



But wait! We also can't prove that there is a bijection between $omega_1$ and $mathbb{R}$, so the rule says there isn't one, and the continuum hypothesis is false.







share|cite|improve this answer












share|cite|improve this answer



share|cite|improve this answer










answered Jan 19 at 4:00









Eric M. SchmidtEric M. Schmidt

2,56111228




2,56111228












  • $begingroup$
    No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
    $endgroup$
    – Andrés E. Caicedo
    Jan 19 at 12:59


















  • $begingroup$
    No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
    $endgroup$
    – Andrés E. Caicedo
    Jan 19 at 12:59
















$begingroup$
No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
$endgroup$
– Andrés E. Caicedo
Jan 19 at 12:59




$begingroup$
No, it is not that simple. The system with the proposed axiom added certainly isn't ZFC anymore, and it is not even first-order.
$endgroup$
– Andrés E. Caicedo
Jan 19 at 12:59


















draft saved

draft discarded




















































Thanks for contributing an answer to Mathematics Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3078680%2fwhy-cant-we-just-add-nothing-else-is-a-set-as-an-axiom%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







Popular posts from this blog

Mario Kart Wii

The Binding of Isaac: Rebirth/Afterbirth

What does “Dominus providebit” mean?