It takes greater faith to believe that an unseen God exists than it does to dismiss it. This is problematic to philosophers and people of faith since you cannot physically confirm that god is really there. As a result, early-medieval philosopher's such as St. Anselm and Ibn-Sina (Avicenna) attempted to rationally prove the existence of god using vastly different methods.
St. Anselm was an 11th-12th-century monk and the Archbishop of Canterbury who was famous for his ontological proof, a philosophical argument for the existence of God. The ontological argument is one of the most fascinating arguments for the existence of an all-knowing, perfect deity. While there are several different versions of this argument, in the end, it exists to show that it is self-contradictory to deny that there exists a greatest possible being — god. Anselm described his argument in the Proslogion
as follows (Note: "something god-like" refers to "that-than-which-nothing-greater-can-be-conceived"):
"Even the fool is forced to agree that [something god-like] exists in his mind, since he understands this when he hears it, and whatever is understood is in the mind. And surely that [something god-like] cannot exist in the mind alone."
In this passage, Anselm is claiming that it is a truth — by definition — that God is the greatest being that can possibly be imagined and that the notion of an all-powerful, all-knowing deity exists as an idea in our minds. Anselm continues his proof:
"Suppose it exists in the mind alone: then it can be conceived to exist in reality; which is greater…Therefore, if that [something god-like] exists in the understanding alone, the very being, [something god-like], is one that which a greater can be conceived. But obviously this is impossible. Hence, there is no doubt that there exists a being, [something god-like], and it exists both in the understanding and in reality."
Put this way, Anselm's claim makes sense: a being that exists as an idea in the mind and
also exists as in idea, in reality, is inherently greater than a being that exists only
as an idea. Therefore, the argument states that if God only exists as an idea, then our imaginations can think of something greater than god. However, this is contradictory to the fundamental idea of a god who by nature is all-powerful and is the
greatest. Therefore, according to Anselm's ontological argument, God exists.
A favourable contrast to Anselm's clever yet unconvincing conceptual argument is early-Islamic philosopher Avicenna's (Ibn-Sina) explanation for the existence of God. Avicenna's proof has nothing to do with conception. To prove that an all-powerful deity exists, he doesn't need the interpreter to think about an abstract greater thing. Instead, he argues from the standpoint that the things we see around us are dependent on one another.
The main concept of Avicenna's argument, known as "The Proof of the Truthful", is that of determinate things. What he wants to do is show that although all the things we experience directly are determined by — and dependent on — something, there is also something else that exists beyond that, and the very nature of this thing guarantees it exists. To do this, Avicenna points out that since a conditional thing on its own could either exist or not exist, it must have some external cause that made it exist. In scientific terms, this would be a catalyst.
Ibn-Sina's proof can be examined internally. I, for instance, am contingent in the sense that I could have failed to exist entirely. More accurately, there was in fact a time where I never existed. The reason I exist is entirely dependent on a single cause or event that brought me here. For example, one could argue that my parents brought me into existence. Expanding this concept further, Avicenna's logic says that the entire physical universe is also entirely dependent on something, everything is contingent. Therefore, the entire universe needs an external cause for it to exist, and this cause needs to be somewhere outside of all contingent things and it cannot be contingent itself. So, the conclusion of Avicenna's argument is there must be an external entity which is the cause for everything, and this entity is a god.
On the surface, both Anselm and Avicenna's arguments for the existence of God are similar. Avicenna's claims that "necessarily existent by virtue of itself", an entity cannot exist. This sounds similar to Anselm's main theme of "that-than-which-nothing-greater-can-be-conceived".
In contrast, however, Avicenna's reasoning is that the domain of dependencies must have a cause that is not dependent on anything because otherwise it would be included in the said domain. This notion is different from Anselm's argument that "that-than-which-nothing-greater-can-be-conceived", must already exist in reality. Otherwise, that same thing that exists, in reality, would be greater.
Simply put, Anselm provided an ontological proof, which is contrasted by what is known as cosmological proof
. While ontological arguments rationalize the existence of God as an idea from the definition of a "god", cosmological arguments rationalize the notion of god as the first cause. Avicenna's "Proof of the Truthful" happens to involve elements of both arguments, however, it is more cosmological than not. Throughout his work, he did not necessarily define god, he instead provided reasoning to derive its characteristics, which allowed him to identify it with God in Islam.
Of the two schools of thought, Avicenna's cosmological "Proof of the Truthful" is much more rational than Anselm's ontological proof. Anselm's entire argument is predicated on the ability to conceive. God is not described as the greatest thing conceivable, instead, he is described as "that-than-which-nothing-greater-can-be-conceived". This is a paradox. Imagine that my argument is that "god is the greatest thing conceivable". One can then ask me to define the limits of the greatest thing ever. Whatever I say in response, someone else can always argue that what I've defined is not really the greatest thing.
The biggest shortcoming of Anselm's argument, however, is not his conclusion that the conception of God must automatically yield the existence of God, rather it is the idea that anybody is actually conceiving of god in the first place. All of Ansel's notions of what God really is are constructed to obscure a specific definition. Throughout his literature, Anselm constantly refers to god as "that-which-no-greater-thing-can-be-conceived". This is not a definition of god in and of itself, it is a challenge for the reader to think for themselves and conceive god from their own understanding, and is therefore completely subjective. These inconsistencies are why I have trouble agreeing with Anselm's ontological proof for the existence of God.
On the other hand, Avicenna's "Proof of the Truthful" captures a reason that underlies an individual's belief in god. Avicenna is attempting to show that when you observe and understand certain propositions, such as the fact that everything could have failed to exist but does still exist for some reason. And it more or less makes sense too: not everything can possibly be determined. There must be something that has to exist, to explain why everything else has ended up existing. And according to Avicenna, that something is a god.