First, let’s be clear that I do not aim, by any chance, to discuss arguments in detail here. As I said in other posts of this blog, I just use this to keep track of my own views or summarize books I read. On the other hand, please reach out to me on the comments or on Twitter with any questions you have about what I write here. Engaging in discussions about a topic is the best way to learn, so thanks in advance if you do that.
Second, I use the material from Reasonable Faith (by William Lane Craig) a lot in this text, but I tried to read the texts that Craig cites whenever I could get access to them. I also disagree with him sometimes. Hopefully, I will write another version of this post one year from now where I engage with other authors, but that’s not the case yet.
The million-dollar question
Leibniz’s question is very well known among philosophers: “Why is there something rather than nothing?”  I have seen different answers to this question over time:
- for Christians (or theists in general), there is something because God is a personal being who decided to create the universe;
- for agnostics (or some atheists as well), they simply don’t know: the reason for the existence of the universe is something that probably humans will never be able to comprehend, or it is just a brute fact about reality, no explanation given; and
- for others, “There is no such thing as nothing. Nothing, as the absence of everything, is just an abstraction, nothing more.” (Stephen Woodford, Rationality Rules YouTube channel, video)
I will try to argue against the option (b), and probably write later against option (c). Let’s first consider Leibniz’s argument.
Even if we take the eternality of the universe as a given, Leibniz affirmed that this would still not explain why it does exist:
“Let us suppose that a book on the elements of geometry has always existed, one copy always made from another. It is obvious that although we can explain a present copy of the book from the previous book from which it was copied, this will never lead us to a complete explanation … since we can always wonder why there have always been such books… What is true of these books is also true of the different states of the world … so, however far back we might go into previous states, we will never find in those states a complete explanation for why, indeed, there is any world at all, and why it is the way it is.” 
For Leibniz, the only explanation for the existence of a contingent being such as the universe is a being “whose own existence is to be explained only by reference to himself,”  i.e. a metaphysically necessary being which people usually call God. OK, let’s unpack this. Leibniz affirms that:
- The universe is a contingent being;
- God is a necessary being;
- Only a necessary being would not require an explanation outside of itself.
The contingency of the universe
The notion of contingent being is better understood if we define possible worlds first: a possible world “does not mean a planet or even a universe, but rather a maximal description of reality, or a way reality might be. … A possible world is a conjunction which comprises every proposition or its contradictory, so that it yields a maximal description of reality — nothing is left out of such a description.”  Then, a contingent being is some being which exists in at least one but not all possible worlds. On the other hand, a necessary being exists in all possible worlds. Leibniz says:
“And so we must pass from physical or hypothetical necessity, which determines the later things in the world from the earlier, to something which is of absolute or metaphysical necessity, something for which a reason cannot be given. For the present world is physically or hypothetically necessary, but not absolutely or metaphysically necessary.” 
Our own intuition can give us some clues about whether or not the universe as a whole is contingent. It is easy to imagine a world where no concrete objects exist. It does not seem to exist a clear reason why that would not be the case. Nevertheless, someone could point that the fundamental particles of the universe would still be the same, but it’s hard to see someone daring to say that “some quarks, though looking just like ordinary quarks, have the special occult property of being necessary, so that any universe that exists would have to include them.”  Honestly, I think Craig commits a fallacy of composition here. Even if each thing in the universe is a contingent thing, this does not imply that the universe as a whole is a contingent thing. In any case, “there is nothing about the universe that implies or even suggests that it is a necessary thing.” 
Trying to be complete in the discussion about contingent and necessary beings, some empiricist philosophers will challenge the use of modal notions and possible worlds: “They have insisted that experience never reveals what is necessarily the case or possibly the case, but only what is the case.”  The problem is that they are ignoring all the knowledge that we can have by reasoning alone, and considering it valid only experiential knowledge. This is a discussion for a post about epistemology, so I’ll leave it for another occasion.
The necessity of God
OK, so why is God necessary? From Leibniz’s argument, it follows that the explanation for the universe’s existence would have to come from something outside of physical reality (otherwise it would be part of the contingency): “there must be an entity whose essence is existence, and therefore something must exist which differs from the plurality of things, which differs from the world, which we have granted and shown is not of metaphysical necessity.” 
What are the options here? Some, such as William Lane Craig, have suggested that this being is either an abstract object, or a soul or mind. However, it is obvious that abstract objects do not stand in any causal relations. “So if the universe has an explanation of its existence, that explanation must be a transcendent, unembodied Mind which created the universe — which is that most people have traditionally meant by the word ‘God.’” 
I know there are a lot of jumps here, but it was not my goal to cover the whole discussion about Leibniz’s argument.
The universe can’t just exist
For people that follow option (b) that I described in the beginning of the text, this is kind of the reasoning: they don’t know why the universe exists, so they often assume that it just does. No reason needed. This is called a brute fact of reality, in philosophical terms.
Another wording for “only a necessary being would not require an explanation outside of itself” is the way William Lane Craig enunciates Leibniz’s premise: “Anything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause.”  This is known as the Principle of Sufficient Reason (PSR).
“For the reason for things must be sought in metaphysical necessities or in eternal truths, since … it cannot be found in the actual series of things. But existing things cannot derive from anything but existing things … So it is necessary that eternal truths have their existence in a certain absolute or metaphysically necessary subject, that is, in God, through whom those things which would otherwise be imaginary are realized.” 
Craig also points that some will challenge this premise since the universe would be an exception of this rule: “the explanation of any physical state of affairs S must be found in a causally prior state of affairs in which S does not exist.”  Since this causally prior state of affairs would be an empty world, and an empty world can’t cause anything, then it is absurd to require an explanation for the existence of the universe.
However, this objection “plainly begs the question in favor of atheism. For unless one assumes in advance that the universe is all there is, there’s just no reason to think that the state of affairs causally prior to the existence of the universe which explains why the universe exists has to be a physical state of affairs.” 
Again, just to be complete on the discussion, it is acknowledged by most who defend the PSR that it cannot be proven, “since it constitutes one of the basic axioms of rational thought against which all other claims or statements are measured.”  The conclusion arrived by Davis is that both the belief in God and the disbelief in God are rational, considering only the General Cosmological Argument (a similar version to what we are discussing here). 
Unicorns also can’t just exist
Another objection to the Principle of Sufficient Reason was raised by Bede Rundle in his book “Why is there something rather than nothing,” published by Oxford University Press. Rundle agrees that it is impossible that nothing exists, but the conclusion from that fact should be: “necessarily, some contingent being or other exists… This is akin to saying that while, necessarily, every object has a shape, nonetheless there is no particular shape which everything necessarily has.” 
In other words, the universe is still a contingent being, but the explanation for its existence is just that there had to be something that exists. You still keep God out of the picture.
In an academic book review about Rundle’s book, Alexander Pruss points out that Rundle’s views have counterintuitive consequences.  If you take all the propositions about the existence of contingent beings other than a unicorn (i.e. everything that is not a unicorn), let’s say p1, p2, p3, …, the conjunction (not p1) or (not p2) or (not p3) … entails that there is a unicorn! 
To be honest, I’m not sure if I agree with Pruss’ counterargument or find it particularly convincing, but I definitely agree that someone would still have to answer the question about why there exists contingent beings in every possible world.
I hope this is useful for some of you. As I stated multiple times, I don’t have any goals of providing a thorough analysis of Leibniz’s arguments, or the responses to it over time. If I were to do that, I would write an academic article. This is only meant to be a dump of what I thought about the subject so far.
Please, leave comments about parts you don’t understand, or about which you think I’m wrong. I’m glad to come back and edit the post if I deem necessary.
 Gottfried Wilhelm Leibniz, “Principles of Nature and Grace, Based on Reason (1714),” in Leibniz: Philosophical Essays, trans. Roger Ariew and Daniel Garber (Indianapolis: Hackett Publishing Company, 1989), Kindle Edition, 206.
 Gottfried Wilhelm Leibniz, “On the Ultimate Origination of Things (1697),” in Leibniz: Philosophical Essays, trans. Roger Ariew and Daniel Garber (Indianapolis: Hackett Publishing Company, 1989), Kindle Edition, 149.
 William Lane Craig, Reasonable Faith: Christian Truth and Apologetics (Wheaton, IL: Crossway Books, 2008), 3rd ed., 99.
 Ibid, 183.
 Leibniz, “On the Ultimate Origination of Things” in Leibniz: Philosophical Essays, 150.
 Craig, Reasonable Faith, 109.
 Stephen T. Davis, “The Cosmological Argument and the Epistemic Status of the Belief in God,” in Philosophy of Religion: A Reader and Guide, ed. William Lane Craig (Edinburgh, UK: Edinburgh University Press, 2002), 89.
 Michael J. Loux and Thomas M. Crisp, Metaphysics: A Contemporary Introduction (New York: Routledge, 2017), 150.
 Leibniz, “On the Ultimate Origination of Things” in Leibniz: Philosophical Essays, 150.
 Craig, Reasonable Faith, 108.
 Ibid, 106.
 Leibniz, “On the Ultimate Origination of Things” in Leibniz: Philosophical Essays, 152.
 Craig, Reasonable Faith, 107.
 Ibid, 107–108.
 Davis, “The Cosmological Argument,” in Philosophy of Religion: A Reader and Guide, 83.
 Ibid, 90.
 Craig, Reasonable Faith, 110.
 Alexander Pruss, critical notice of Bede Rundle, Why There Is Something Rather Than Nothing, Philosophia Christi 7 (2005), 210.
 Craig, Reasonable Faith, 110.