Agency begets structure
Probably the easiest illustration of this difference is with teleological arguments. Take the design argument, the familiar argument being with finding a pocket-watch and realising there's a watchmaker. Now for non-believers his argument doesn't hold because watchmakers don't exist ex nihilo, the abilities to make watches has come from thousands of years of R&D resulting in an accumulation of knowledge allowing for the making of such watches.
But to a believer, God just is by very definition. He's all-powerful so he has the ability to make stars and starfish, he's all-knowing so he has the knowledge to do such things. So while the modest pocket-watch for humans may have required millions of hours of R&D before it could be made, the very definition of God entails the ability to craft such elaborate structures.
But, as a non-believer would counter, knowledge again doesn't exist ex nihilo but is housed in the elaborately ordered and complex brains of humans. Anything that is capable of having knowledge to create structure must itself have been created. Positing an agent of sufficient ability to be one that begets structure must itself have been created.
God is the uncaused cause, a believer would counter, and not something built of material components. God just is by pure definition, if God needed a cause or was physical then it wouldn't be God. Thus the critiques levelled at God miss the mark because they fail to grasp the nature of the concept of God.
How do you know?
The above mock exchange is to highlight the difference between the argument. I don't see that anything attributed to God in any meaningful sense can be a priori, especially if as they say God is supernatural. it puts any attribution of anything down to pure speculation, unverifiable and almost certainly anthropomorphic.
While some might use a priori proofs to reason about the nature of God, all it could ever possibly do is show that the concept is ontologically valid. To think about this another way, consider the possibility of unicorns. Now if we can conceive of unicorns as being logically valid entities, does it follow that great leaders rode unicorns into battle? Or even that unicorns even exist as anything more than a mental construct?
Any qualities attributed to any a priori concept can only be a posteriori. What does it mean to say that God is omniscient? How can we judge the quality of having knowledge, let alone all knowledge, without putting it in reference of terms we understand from our personal experience?
There's just no justification for any a priori arguments, any potential logical axiom is underpinned by induction. That one can follow a premise through to a conclusion doesn't suddenly validate the premise, the knee-jerk dismissal of philosophical arguments that some atheists do is unfounded.
A prime mover
A prime example of this is the first cause argument, which can be framed as follows:
- Every finite and contingent being has a cause.
- Nothing finite and contingent can cause itself.
- A causal chain cannot be of infinite length.
- Therefore, a First Cause (or something that is not an effect) must exist.
The first premise is simply unfounded, it's an a posteriori argument. How do we know that every finite and contingent being has a cause? All one would have to do is say that the first premise fails and the whole cosmological argument falls apart. There's no need to even follow it to the completely unjustified position that it was the God of the Bible and thus Jesus died for your sins.
There's really not much more that needs to be said. Such arguments that can best be described as pseudo-intellectual mental masturbation, meanwhile physicists are creating and testing theoretical frameworks in order to understand the universe we reside in. What of ideas such as a multiverse or quantum foam? What of that particles can come into and then go back out of existence? What about the big bang being the creation of time and thus the question of a prime mover becomes incoherent?
This is not to dismiss arguments made a posteriori, rather to show that the arguments made for God's existence are a posteriori arguments. This really shouldn't pose a problem for believers, after all they are talking about an interventionist deity who is meant to be active in our world. This post was meant to illustrate the gap I find when trying to have an argument with those who are looking for an a priori argument to ignore or invalidate any argument based on evidence / experience.
[T]here is an evident absurdity in pretending to demonstrate a matter of fact, or to prove it by any arguments a priori. Nothing is demonstrable, unless the contrary implies a contradiction. Nothing, that is distinctly conceivable, implies a contradiction. Whatever we conceive as existent, we can also conceive as non-existent. There is no being, therefore, whose non-existence implies a contradiction. Consequently there is no being, whose existence is demonstrable. - David Hume (Dialogues Concerning Natural Religion)