How Do We Prove the Existence of God?

The Rationalist Proof (As adapted from Edward Feser's "Five Proofs of God")

[1] All things are either Necessary-in-themselves or contingent. [2] Define "The Universe" as the collection of all contingent things. [3] It follows that "The Universe" must then also be contingent. [4] Therefore, "The Universe" requires a cause. [5] The cause of the Universe is either contingent or Necessary-in-itself. [6] If the cause is contingent, it is included in "The Universe," and is then itself in need of a cause. [7] An infinite regress of contingents is impossible. [8] Thus, the cause of "The Universe" cannot be contingent. [9] So the cause of "The Universe" must be Necessary-in-itself. End. 

Explanation

[1] All things are either Necessary-in-themselves or contingent. 

Things exist. This is obvious. But it's also obvious that things don't simply pop into existence for no reason. This would be absurd. Rather, we believe things need explanations. This belief is called the Principle of Sufficient Reason (PSR). 

If we denied this belief then, for example, we could not properly do scientific inquiry. All science is based on belief in the PSR. This is because science investigates the explanations of phenomenon. Imagine if a group of stubborn scientists who denied the PSR saw a beam of light in the sky and threw their hands up and said it happened just because it did, and that it has no fundamental explanation. Or for example, imagine if a scientist ran an experiment by mixing rust and aluminum powder with each time resulting in a violent, hot reaction, and he simply threw his hands up and claimed there was no explanation for the hot reaction and no relationship to the mixing whatsoever. We would call these scientists silly, and tell them to do their job better. But this is exactly the kind of consequence that denying the PSR has. Without the PSR, we could say that if a flash of light happened, it is not necessary for there to be an explanation for it, and that it could have happened at random. And similarly, if the violent, hot reaction happened, it is not necessary for it to have any relationship to the experiment or to have had any cause at all! There are other, more absurd, consequences to denying the PSR which can be realized by thinking deeply enough about the principle, but the fact that denying the PSR undermines the assumptions of scientific inquiry is enough of a consequence. 

The reason we demand explanations for these things is because these things are contingent. When we say they are contingent, that means we realize that they could have been otherwise. We don't simply accept things popping into existence, or that things exist in a particular way for no reason at all, because this would be arbitrary and against our most basic intuitions. Hence, we demand an explanation for these sorts of things. 

So to be more formal and precise with our definitions, a thing is contingent if it is in need of an explanation. A thing is Necessary-in-itself if it is a wholly sufficient explanation in of itself and explains contingent facts- that is to say, its existence is required to explain all other contingent things, and that if it didn't exist, we could not have an explanation of anything after it. God is the Necessary-in-itself. If we can find the Necessary-in-itself through our argument, then we have succeeded. 

In other words, we say things have causes. God is the uncaused causer. If we can go from the idea that things have causes to finding the uncaused cause, then we have succeeded in proving God's existence.

[2] Define "The Universe" as the collection of all contingent things. 

We can also call "The Universe" the "Big Collection of Contingent Facts" or the "BCCF."

I chose "The Universe" as a name, because the material Universe as we know it is a contingent thing. Yes- all material existence is contingent. 

There are many proofs that the Universe is contingent. The most simple proof is that we recognize the Universe could have not existed, but yet it still does exist in the way that it does with its physical properties and constants and so on. And since we don't accept that things don't pop into existence arbitrarily or exist the way they are for no reason at all, we would demand an explanation for the Universe. 

Another proof that The Universe is contingent is that The Universe itself is changing. The Universe, for example, is in a state X in the beginning, at time T1, then it changes to Y at T2, Z at T3… and so on. X existed and then did not exist. X is a state of affairs that depends on some factors to exist, because it may or may not exist. By definition, this is contingent- X has an explanation as to why it is as such (whatever factors existed as to why The Universe was X at T1). (See also these hadith where the Imam (a) discusses this proof: [Al Kafi] [Al Tawhid])

So whatever the explanation or cause of "The Universe" is, it had better not be material, or changing. 

[3] It follows that "The Universe" must then also be contingent. 

This follows trivially from [2]. It is obvious that if every member of the set is contingent, then they are contingent. 

[4] Therefore, "The Universe" requires a cause. 

If "The Universe" is contingent, then as we had defined, it is in need of an explanation/cause. And the Universe is contingent, so therefore it requires a cause.

[5] The cause of the Universe is either contingent or Necessary-in-itself. 

As we had already set out in [1], all things are either contingent or Necessary-in-itself.

[6] If the cause is contingent, it is included in "The Universe," and is then itself in need of a cause/explanation. 

This follows trivially again from [1] and [2]. 

[7] An infinite regress of contingents is impossible. 

The simplest intuition for this is to imagine for the sake of argument that there were an infinite regress of causes. If one were to imagine a random point “within” the chain and attempted to go trace to the present, it would be impossible to do so, since the present is infinitely far away from any point within the chain. But this is absurd since we exist in the present now, so an infinite regress of causes could not have been the case. We know that there has to be a finite number of steps from any point we choose in the past in our chain to the present. 

Another intuition for this fact is that if we assumed that an infinite regress was possible for a series of contingent causes, then we would reach no explanation at all, and then every member of the chain would be left unexplained. For example, imagine we had a tower with an infinite number of floors. What would the tower be resting on? Of course, we would say the top floor rests on the floor below it, and the floor below it rests on the floor below that, and so on. But if we said there was no bottom floor at all, then we would be saying the tower rests on nothing at all, and shouldn't even be standing in the first place! Obviously, this is unintuitive, and the intellect deems such scenarios impossible. 

[8] Thus, the cause of "The Universe" cannot be contingent. 

If it were contingent, it would result in an infinite regress of causes and leave "The Universe" unexplained. According to [7] an infinite regress is impossible. So the ultimate cause cannot be contingent. 

[9] So the cause of "The Universe" must be Necessary-in-itself. 

This follows trivially from [1]. If the cause isn't contingent, it must be Necessary. So the cause is God, the Necessary Being. 

And this marks the end of the proof. 

Objections to the Rationalist Proof Answered

Objection 1: If everything requires a cause, then what caused God?

Response: This objection misunderstands the proof put forward. We do not commit to the claim that everything requires a cause. We are saying that every contingent requires a cause. Something is contingent if it can be otherwise, whether that means it could have not existed or if it could have existed differently than what it exists as. But something Necessary cannot be otherwise: its existence is required, and not only is its existence required, but it must be the way that it is. This means the Necessary Being's existence actually explains itself fully by its very essence, and so we do not have a problem where we selectively apply the principle that all things require explanations to only contingent truths, since the Necessary Being's existence is self-explanatory because it cannot be otherwise. So, by definition the Necessary Being cannot have a cause. 

Objection 2: Doesn't quantum mechanics (QM) refute the PSR since QM says certain effects are random?

Response: QM breaks the PSR no more than rolling dice breaks the PSR--which is to say: it does not phase the argument from the PSR at all. The PSR says that contingent things must have explanations because contingent things can be otherwise. The PSR does not say that causes are required to produce a certain effect. Likewise, the PSR does not say that explanations are given for why one effect was reached rather than another. (See Alexander Pruss "Contrastive PSR"). So for example, if a die rolled 6, the explanation is that the die was rolled, and the die had a capacity to roll on 6. All that the PSR demands in this situation is that the die landing on 6 required some explanation, and not that it magically appeared on 6 without anyone rolling it. Similarly, say, for the sake of argument, the position of a photon can only be described by a probability distribution. So if a photon lands on a certain position in an experiment, then this is no different at all from rolling a die. The fact that the photon landed in a particular place is fully explained by the light being shone and the photon having a certain probability to land in the area that it did. All the PSR demands in this situation is that the photon did not end up where it did out of thin air. 

Objection 3: Why can't the Necessary Being just be the Universe itself?

Response: As we demonstrated above, the Universe is contingent. The Universe does not have to exist, or exist the way it does, and the fact that it is changing or has the capacity to change is the clearest demonstration of that fact. 

Objection 4: "God of the Gaps"

Response: The "God of the Gaps" argument is that there are things which we do not have scientific explanations for, and because we cannot find one, one must not exist in the realm of observable reality, and therefore the cause of those things must be God. The argument from the PSR is not saying explanations for things do not exist, it is saying exactly the opposite of that. Nor does the argument from the PSR proceed from the premise that we cannot find scientific explanations for things. In fact, the PSR welcomes scientific explanations for physical realities. So even if scientists explain every possible physically observable phenomenon through scientific explanation, still this would not refute the proof from the PSR. To make the point clearer, consider the following example:

• "God of the Gaps" would hold: (1) It is raining, (2) We cannot explain it => The explanation must be God.

• The proof from the PSR would hold: (1) It is raining, (2) Things do not happen arbitrarily (PSR) => There must be an explanation. 

Proofs of the Attributes of God (As adapted from Robert Koons' "Why the First Cause Must be God")

Unity

There must only be One God. This is because of the Law of the Identity of Indiscernibles. The Law of the Identity of Indiscernibles states that there cannot be two distinct beings with all exactly the same properties. So for example, it is impossible to say that there exist two benches, but they are the same in every single way, including location, size, properties, etc, because they would simply be identical to the other. As we will soon demonstrate, the Necessary Being (i.e. God) must have certain properties, like Omnipotence, Omniscience, and so on. Because God must have these properties, it is impossible for there to be two Gods, since they will simply have the same properties and would therefore be identical. 

Simplicity

The Necessary Being, God, must be a simple Being. The meaning of simplicity is that God is not composed of parts or different distinct attributes. Rather, God's Essence is identical to His Attributes. But before we prove that God is simple, we must first look at contingency in a different way. So far, we've been saying that a thing is contingent if it can be otherwise, and its possibility of being otherwise demands an explanation. Another way of saying this is that a thing's possibility of being otherwise means that it does not have within itself an explanation for its existence--that is, it depends on another for its existence, and derives its existence from another. If this is true, then the PSR proves God is absolutely simple. As we will soon demonstrate God has certain qualities: omnipotence, omniscience, etc. Simplicity means that these qualities are not distinct in God, and they are not separate from God. The proof for God's simplicity is that if these qualities were separate from God, then these qualities would derive their existence from God, making these qualities contingent. So for example, if God and His Power were separate existences, then God's Power would depend on God. Therefore, there is no escape from the position that God in Himself must be powerful to be able to bring about contingent things. Or more generally, it is impossible to escape from the position that God in Himself must have all of the qualities that God has. 

Omnipotence

God must be omnipotent. Recall that a contingent thing can be otherwise. So if someone claimed the Necessary Being was only powerful enough to create the particular universe we are in, then this would mean the Necessary Being is contingent. We can obviously imagine the Universe having different properties and qualities, and likewise if God were only sufficiently powerful to create this particular Universe, we could imagine God having different properties. Hence, because God is the Necessary Being, God must be fully powerful to create all possibilities without any room for arbitrary limitations. 

Life

God is not an inert, lifeless, and dead Being that created the Universe and let it be. Rather, God is constantly involved in the maintenance and direction of the Universe and its affairs. Once we've acknowledged that the Universe is contingent and therefore derives its existence from something other than itself, with all contingents deriving their existence from God, then it becomes clear that a contingent cannot have an independent existence. This means, not only is God required for the existence of a contingent thing, but God is also required for maintaining the continued existence of any contingent thing.