"If you think God just exists without a cause, why can't the Universe just exist without a cause?"
There are several different problems with this commonly heard response.
One is that it fails to be responsive to the nature of cosmological arguments. As I've said before, any cosmological argument will explicitly state some property P of things which need a cause, and conclude to the existence of something without that property P.
A second issue is that in traditional Natural Theology, proponents of cosmological arguments don't usually think that God "just exists" in the sense that he's a brute fact who freakishly exists, randomly and for no particularly good reason. Rather, God is a necessary being who has to exist, by the very nature of "God" and "existence". In this viewpoint, although God is uncaused, there is still a very good reason for his existence! (And according to traditional Natural Theology, anything which "just exists" in this strong sense, would need to have a bunch of other divine properties as well.)
Suppose you think that some things have a cause and some things don't. Then it seems reasonable to think that whether or not a thing needs a cause depends on what kind of thing it is.
But as soon as you admit this, you are committed in principle to accepting some sort of cosmological reasoning, even if you don't think the proper conclusion is Theism. If Type A things don't need a cause, but type B things do need a cause, then it seems to me that if you work out these principles in a sensible way, you're ultimately going to end up with a hierarchical picture of the world: in which the type B things depend in some way on the type A things to exist, but not vice versa. This implies that the existence of type B things should testify in some way to the existence and nature of the type A things. And that's a cosmological argument!
Can we avoid this kind of reasoning by saying that "The Universe just exists"? (Thus putting the term "Universe" in the category of type A things, that don't require an explanation.)
In my opinion, that statement is fatally ambiguous, so that it simply can't be accepted as meaning anything definite, unless further clarification is given. Even if Naturalism is true, the cosmological argument demands a more complicated response than that.
The problem I want to highlight here arises from a very simple, and I hope uncontroversial fact:
"The Universe" refers to a collection of multiple things.
Of course, there are lots of collections of diverse entities which the mind can consider — your uncle's stamp collection; the ecology of all living species in the planet Earth; the natural numbers; the United States of America; the Grand Canyon; a deck of playing cards; a nuclear family; the "set" consisting of my left Big Toe and the Washington Monument; etc. Some of these entities have a natural or artificial "unity" that makes them function in certain definite respects as a single entity. Others, are just random collections of entities, which we can still name linguistically as a collection, if we have some reason to do so.
Now the Universe is a really, really big collection of things. In fact it is defined to be the collection of everything — or more precisely, everything that exists in the particular way that physical things exist.
The caveat is important. Some might prefer to define the "Universe" to consist of everything that exists in any way whatsoever. But then even God would be part of the Universe (assuming he exists at all, he exists in some way, even if that way transcends the way any of us exist). And that would mean that an atheist couldn't express their Atheism by saying "Only the Universe exists" (because saying that wouldn't exclude God), nor could a theist express their Theism by saying "God created the Universe" (because God didn't create God).
Since such an unrestricted definition of "Universe" would be annoying for both theists and atheists, I take it that it is bad form to define it that way. "God" and the "Universe" are sufficiently different that they can't be reasonably regarded as belonging to the same category of entity. (Indeed, in a Monotheistic worldview, God is so unique that he can't really be categorized into a common category with anything else.)
Someone might therefore reasonably propose the following definition:
The Universe is the collection of every entity that exists in space and time.
(Some Naturalists might quibble about whether this "spacetime" criterion is the best way to pick out the class of entities they believe in; others might welcome it. Most of what I say below won't depend on that. The most important thing here is that the Universe is a collection. And that now, if you say "The Universe is the only thing that exists", you are making a substantive claim, that could be right or wrong.)
Now, supposing someone claims "The Universe just exists", my question for them is this: what does it mean for a collection to "just exist"? I would like to argue, that in order for an assertion about a collection to be a meaningful statement, it has to be made clear how the assertion relates to the individual members of the collection.
For example, America is a collective noun referring to a political union which (in addition to being a government, and the territory in which that government has legal jurisdiction) is also a specific collection of individual humans: namely those with American citizenship.
So if a political commentator says "We need to prevent America from dying", there are multiple ways this statement about the collective might relate to individuals. The political pundit might be afraid of any one of the following outcomes:
(A) that all individual Americans are about to die (e.g. from a nuclear war or meteor strike that leaves no survivors);
(B) that some particular subset of individual Americans are about to die (e.g. from Covid-19);
(C) not that any specific American is about to die, but that some property held in common by many American individuals (like the "American spirit" or "American project" or "American respect for the Bill of Rights) is about to perish (e.g. if the country were in danger of becoming a one-party totalitarian state).
(D) that the United States federal government (i.e. the union which stands for Americans collectively) will formally cease to be (e.g. if all 50 states were to secede simultaneously, or if the government disbanded after being conquered by another country).
Of course a patriot would regard all four of these interpretations as bad news, but they are logically distinct meanings, even if some of them might imply or lead to each other. A pundit who confused these outcomes with each other, would be guilty of mushy thinking.
So now let's consider this mushy slogan:
"The Universe Just Exists"
What does it even mean? It seems to me that the options are similar to the case of America dying. If you endorse this slogan, do you mean:
A) Every single entity in the Universe (e.g. rocks, trees, cats, galaxies, etc.) just exists, without any cause.
This interpretation has the advantage that, if it were true, it would be very reasonable to summarize it with the slogan: "The Universe Just Exists".
But it is also manifestly absurd. If we pick this option, then nothing has an explanation, nothing makes any sense, and nothing we do can make any difference to anything else. And yes that would also mean that Science is impossible.
B) Some specific entity or entities just exists, without any cause.
OK, in that case you need to tell me which specific entities these are. In other words, you have more work to do before you've succeeded in expressing an actual philosophical position.
Once you have found whatever thing(s) X you think `just exists', it seems to me it would be much clearer to state your position as "X just exists", not "the Universe just exists". After all, on this hypothesis, the Universe contains many other things, which are not X's.
If you pick this option, it had better be plausible that your X is able to give rise to all of the non-X's, without any features left over which X is unable to explain. (And it would be nice to have an account of which specific features of X, as compared to not-X's, make it reasonable to think they don't need a cause.)
C) No specific entity just exists without a cause, but something which these entities have in common just exists without a cause.
Again, you have more work to do here. You need to tell me what specific common factor F you believe "just exists". And whatever this thing is, since you've distinguished it from all the collected members of the class "Universe", it's unclear that your position will be best summarized by the phrase: "the Universe just exists". You should say "F-ness just exists".
Of course, if you thought that F was a concrete entity existing in space and time, it seems you should have said "yes" to option B, rather than select this option. To avoid (B), you are forced to pick one of the following 2 options:
1. the thing which "just exists" is not an entity at all, or
2. the thing which "just exists" is an entity that exists outside of spacetime
This puzzle is related to the fact that things which are held in common tend to be abstract qualities, e.g. both apples and fire engines can be red, so redness is an abstract quality rather than being a concrete entity. If you are a Platonic or Aristotelian "Realist" who believes that abstract universals really exist in some transcendental or immanent way (a position I have some sympathy with), then maybe this is okay. But if you are a hard-nosed Nominalist who doesn't like forms, then you might be in danger of implying this:
the factor F which "just exists" (and which explains everything else), doesn't actually exist at all in the way normal things do (except in a sort of purely conventional or descriptive way)
that which explains everything else by its existence, doesn't exist except in our minds
or more provocatively still:
that which exists most, exists least
which seems like a bit of a paradox!
Incidentally, if you think that the reason why the Universe doesn't need a cause is because of some infinite causal regress, then your position is probably best thought of as an example of position (C). To give an example, an oak tree usually grows from an acorn dropped by a previous tree. Darwin would say that trees evolved from simpler life forms, while Young Earth Creationists think the first oak tree was immediately produced by God without forebears. But suppose, contrary to both of these views, that the Universe were infinitely old and that oak trees go back in time forever, in an infinite regress. Then in this silly hypothetical, there would be a cause for why any particular oak tree exists, but it would seem there is no cause of why oaks as a species exist, nor is there a cause of why the arboreal nature which is common to all trees is instantiated in the world.
Yet the existence of oaks is still a rather peculiar and interesting fact, which we might like to have an explanation for.
If a Naturalist thinks that the most basic reality is the laws of physics (as I've previously suggested is the most plausible form of Naturalism), then this might also be regarded as a form of option (C). Note however, that the laws of physics would transcend space and time (in the sense that they hold equally in all times and places) and on this view they would also seem to transcend the distinction between abstract reality and concrete reality (since they take the form of mathematical equations, but they also govern the universe).
D) No specific entity exists without a cause, but collectively they form a united whole which, considered in this collective identity, exists without a cause.
This option was supposed to be parallel to the "United States government" option for "America", but I confess I'm have some difficulty making much sense of it as applied to the subject "the Universe" and the predicate "just existing".
First of all, it is not clear whether the Universe, which is after all just about the broadest category there is, has any collective identity above and beyond the coexistence of the individual entities which exist within it.
If it does have some collective identity, it's hard to see how that collective identity doesn't depend on the individual parts to exist. If you try to make a brick house, but you don't have any bricks, then you won't be able to build the house. But if something depends on other things to exist, then it doesn't "just exist", now does it?
(Otherwise, if the identity of the Universe didn't depend on the parts in any way, such that it could exist even if none of its parts do, then maybe the thing you have in mind isn't a collection of entities at all, and you should have called it "God" instead of "Universe"?)
Of course if you just replace a few of the bricks from a house, you could still have something that might recognizably be considered the "same" house. Just as individual Americans can die (in fact everyone who was alive at the founding has died, and been replaced by new generations) without the United States of America ceasing to exist.
But, if it is coherent to say that the USA of 1790 is the same as the USA of 2020 (or a counterfactual USA which lost the Civil War and had fewer states), then it must be true that there is some abstract feature, which is common to all of these versions of the USA, and which makes them the same.
Similarly, we could imagine the history of the universe turning out differently. E.g. imagine an possible world W' in which the Solar System had never formed. We can now ask, does W' count as the same "Universe" for purposes of the slogan, as the actual world W?
(To some extent this is an ambiguous question. You can define "this Universe" however you like, and there may well be fuzzy boundaries about exactly when normal people would count a universe as the "same" or "different". But the relevant question is this: is there any specific way to define "the Universe" that makes the slogan "the Universe just exists" true?)
If W and W' count as distinct Universes, then it is neither W nor W' which you are specifically asserting "just exists", but rather some common aspect which is the same for both W and W' — and that pushes us back into option (C), or maybe (B) if there are some spacetime entities which exist in all possible worlds.
If on the other hand, we interpret "Universe" in the slogan sufficiently narrowly that any changes to W would make it no longer count as the same universe — then in that case asserting that the Universe just exists, means asserting that W obtains as a brute fact, where W implies that every single thing in the universe is exactly the way it is. But in that case it seems that the slogan implies (A), which is still absurd.
In conclusion, every single interpretation of "The Universe Just Exists" that I can think of seems to be either absurd, or else it uses the word "Universe" to refer to something far more restricted and definite than what it is normally taken to mean.
Thus, my message for Naturalists is simple:
1. The Universe can't "just exist"
2. So something else must exist without a cause
3. Come back when you think you know what it is,
and then we can have a real conversation about the