A common formulation of Leibniz’s Cosmological Argument is as follows:
1. Everything that exists has an explanation of its existence (either in the necessity of its own nature or in an external cause).
2. If the universe has an explanation of its existence, that explanation is God.
3. The universe exists.
4. Therefore, the universe has an explanation of its existence [1, 3].
5. Therefore, the explanation of the universe’s existence is God [2, 4].
Since this is a logically valid argument (that is, it follows a proper form known as modus ponens), if one wishes to avoid the conclusion then she must deny at least one or more of the premises. (3) is true so long as one is being rational. Moreover, consider that the first premise, also known as the Principle of Sufficient Reason, is highly intuitive. Some may retort the PSR is highly controversial, but I suspect the main reason it’s so is that people wish to avoid the conclusion of a God, not because things or state of affairs being explained is so controversial.
As Craig notes, this PSR is wholly compatible with there being brute facts about the world. This undercuts the major objection to the PSR. He goes on to note that “explicability is the default position and that exceptions to the principle therefore require justification.” That justification doesn’t seem to be forthcoming. Moreover, as Groothuis objects, it seems quite ad hoc to require explanations for existence and states of affairs in science and other areas of life but not as it relates to cosmology or the universe. In fact, it remains literally inexplicable why just anything exists if the PSR is false, ultimately.
Perhaps the skeptic or atheist may wish to accept the PSR (as seems eminently plausible) yet reject (2), or perhaps they reject this in addition to rejecting the PSR. They may wonder just why they should accept that God is the universe’s explanation, if there is one at all. First, one must note the contrapositive of the premise: If the explanation is not God, then the universe has no explanation. This is a line repeated by more than one atheist (including Bertrand Russell, who famously thought there simply was no explanation for why there is something rather than nothing). But since any premise’s truth-value necessitates the same truth-value for its contrapositive, atheists who take Russell’s track are already committed to the truth of (2)!
Perhaps an objector may wish to claim the universe is necessary rather than contingent (and hence affirm  and deny ). But in that case, not only must the universe exist from an infinite past, but also must exist into the infinite future. That is to say, the universe cannot fail to exist, or cease to exist; there cannot be a state of affairs of nothingness as it relates to the universe. But that is a radical claim. Since we can conceive of the universe’s non-existence, what non-arbitrary reasoning can there be for affirming its necessity?
Next, Groothuis shows reason to reject that the universe just exists without an explanation and that it is necessary. “The metaphysical implication of rejecting the principle of sufficient reason with respect to the cosmos is that the cosmos is meaningless . . . nothing has any ultimate meaning, and . . . everything is gratuitous.” Essentially, this amounts to nihilism. Further, one must reject the universe as a “being that explains itself” because of the highly-plausible conception that the universe can or could have fail/failed to exist. “If the universe were . . . self-explanatory and self-existent, such a question [why is there something rather than nothing?] would be radically out of place—on the order of asking, Why is the law of noncontradiction true?” But it is not so radically out of place.
In conclusion, it seems the PSR is extremely intuitive, the universe exists, and it does not explain itself. But if that is the case, then a final self-explanatory being is in mind; exactly what we call “God.” We may then conclude the God of monotheism is the best (and really, only) candidate for this being. From that, Christian evidences can and should follow. What a great argument!
 William Lane Craig, “Argument from Contingency,” http://www.reasonablefaith.org/site/News2?page=NewsArticle&id=5847, accessed September 19, 2011.
 William Lane Craig, Reasonable Faith, 3rd ed. (Wheaton, IL: Crossway, 2008), 107.
 Douglas Groothuis, Christian Apologetics (Downers Grove, IL: IVP Academic, 2011), 212-13.
 Ibid., 212.
 Ibid., 213.
 Craig, Reasonable Faith, 99.
All posts, and the blog Possible Worlds, are the sole intellectual property of Randy Everist. One may reprint part or all of this post so long as: a) full attribution is given (Randy Everist, Possible Worlds), b) all use is non-commercial, and c) one is in compliance with the Creative Commons license at the bottom on the main page of this blog.