It's all about the deep questions.
Theism: The Argument from Contingency
1. A contingent being (a being that if it exists can not-exist. For example, I am a contingent being because it is possible that I not-exist) exists.
2. This contingent being has a sufficient cause of or explanation for its existence.
3. The cause of or explanation for its existence is something other than the contingent being itself.
4. What causes or explains the existence of this contingent being must either be solely other contingent beings or include a non-contingent (necessary) being.
5. Contingent beings alone cannot provide an adequate causal account or explanation for the existence of a contingent being.
6. Therefore, what causes or explains the existence of this contingent being must include a non-contingent (necessary) being.
7. Therefore, a necessary being (a being that if it exists cannot not-exist) exists.
As anyone can see, for this argument to go through, many premises have to be accepted, so naturally, this argument is open for many criticisms.
Criticism 1: the argument does not prove theism, even if successful
Even if every nuance of the argument was accepted, one would still not be entitled to be a theist. All the argument does prove is that there exists a necessary being, which only establishes property (c). The theist must provide additional argumentation to show that this necessary being is the traditional god of monotheism. Why can’t the universe exist necessarily? Why can’t an eternal multiverse exist necessarily, where our “universe” is simply a bubble universe, consistent with the predictions of string theory? There is absolutely no reason why one would should expect this being to be a person, or all loving, or any of the other properties traditionally given to God. This argument does, however, add some credibility to theism, and that is why other criticisms should be mounted against the argument.
Criticism 2: the argument is self-contradictory
A necessary being is one who exists in all possible worlds. If the conclusion of the argument is sound, then a necessary being exists in all possible worlds. Furthermore, this necessary being is supposed to provide a sufficient explanation, intrinsically, for a variety of contingent beings, which in turn themselves provide sufficient explanations for other contingent beings etc. For x to be a sufficient explanation for y, if x exists, then y must exist. Let x be the established necessary being, and let y be any contingent being that exists in the actual world. Since, at bottom, the necessary being is the sufficient explanation for all contingent beings, then wherever x exists, y also exists. Since x exists in all possible worlds, however, it follows that y exists in all possible worlds. The definition of a contingent being, however, is a being who exists only in a proper subset of all possible worlds (that is it exists in some worlds but not others). The contradiction is established then. y exists in all possible worlds, yet y does not exist in all possible worlds.
The conclusion of the argument contradicts premise 1, given that every contingent being has a sufficient explanation grounded at bottom in a necessary being.
Criticism 3: The argument’s conclusion, if taken to be a theistic god, contradicts property (h) of god.
Most theists define god as having a sort of libertarian free will, as opposed to a compatibilist free will. It is this kind of free will, therefore that I will be addressing. According to the argument, the necessary being (let’s call it god for now) is a sufficient explanation for some contingent being y. The existence of god, on it’s own, isn’t sufficient for the existence of something independent of god, however. God must will it into existence. For god to be a sufficient explanation for y then, god must will y into existence in all possible worlds. This contradicts the power of god to be free however. For god to be free, he must have the ability to do otherwise. Since, however, god wills y into existence in all possible worlds, god does not have the ability to do otherwise. Therefore god is not free, and therefore the necessary being that the argument establishes cannot be the god of traditional theism.
Criticism 4: Premise 5
Why can in not be the case that a successful explanation of contingent beings cannot reference only contingent beings? Consider the explanation for my existence. I exist because my parents exist and decided to have children; furthermore a biological account can be spelled out as to why a specific sperm met an unfertilized egg. That explanation did not make reference to any necessary being, yet it seems to be a satisfactory explanation. The theist here will probably object that one hasn’t explained the existence of my parents, so the explanation is not a complete explanation. Once I give an explanation of my parents, they will then want an explanation of the explanation, ad infinitum. Lastly, even if the non theist points out that there really could be an infinite chain of explanations, the theist asks for a reason for the chain itself. The theist will only be satisfied until god is appealed to as the necessary being that is the explanation for everything. Here the non-theist has two responses, each being plausible. One is that at bottom, some things simply have no explanation. Take the universe for example. In the words of Bertrand Russell, “The universe just is”. Our very concept of “cause” and explanation comes from observing things that occur within the universe. Applying that principle (that everything must have a sufficient explanation) to the universe itself would be to commit the fallacy of composition. Here the definition of the universe is something along the lines of the set of all physical things. (If a kind of multiverse theory turns out to be true, than the multiverse would be “the universe”.) Just because the parts of a whole have a particular property does not mean that the whole has that property as well. Consider the set of all humans, the human race, which is entirely composed of humans. It is true that every human has two parents. Is it true that the human race has exactly two parents? Obviously not. Admitting that the universe is a brute fact should not be all that uncomfortable to theists anyway; their god seems an awful lot like a brute fact. Furthermore, quantum mechanics may provide some credibility to the claim that something’s simply do not have sufficient explanations. On some interpretations of quantum mechanics, some events do occur truly randomly without a sufficient explanation, such as radioactive decay and the existence of virtual particles). Another route that the non theist has is the route pursued by Quentin Smith. In his route, he seeks to establish an infinite chain of explanations. On his view a sufficient explanation for a particular state of the universe at time x is another particular state of the universe, with a completed physics for the “laws of nature”, at any time before x. This is because the universe at time x can be fully deduced from any previous time of the universe along with the “laws” of nature, provided that the laws are deterministic (I put laws in quotes because I tend to take an empiricist take on them). Furthermore, there is no first state of the universe even on the big bang model, since the singularity represents an ideal that cannot be realized (infinite density, curvature, etc.) Therefore, the time line that the universe has passed through at any time t can be represented by the half open interval (0, t]. Since every point in time can be sufficiently explained by reference to an earlier point in time, and for any point in time there is always an earlier one, it follows that nothing in the universe is without explanation. If the theist wants an explanation for the existence of the universe as a whole, David Hume’s response that once the parts are explained, the whole is explained can be utilized.
Criticism 5: trouble with the very concept of a necessary being.
This criticism will be addressed when the ontological argument comes up, which revolves around necessary beings. Kant argued famously against the ontological argument, and for similar reasons he faulted this particular argument.
Conclusion: Seeing that the argument does not even establish theism, the non theist has little to worry about it (in fact, if one accepts the cogency of criticism 3 it is impossible that the necessary being that it describes be the traditional god of monotheism). Furthermore, the argument even contradicts its first premise if accepted! The principle that it relies on, the principle of sufficient reason, seems too strong because it makes everything exist necessarily. But certainly I can imagine a possible world where I do not in fact exist. Therefore, the principle must be flawed. Several distinct approaches are available to the non theist anyway, may it follow Russell, Hume, Quentin Smith, or other philosophers. In conclusion, the argument is utterly unconvincing.