Debating the existence of god with theists almost always comes with one apparent caveat: The criteria that each of you use in inferring ontology is often different. Many theists and atheists simply use different methodologies when trying to make a case for the existence or non-existence of any particular deity, and they often disagree on what they consider "good evidence." This is why evidentialist apologists like William Lane Craig tend to be so popular.
Evidentialism doesn't really assume any epistemological theories dramatically out of line with what most atheists already adhere to. In this sense, evidentialist apologists are trying to compete with the atheists on their own terms by
not appealing to faith, revelation, or scriptural authority to make their arguments for god.[1] Almost all atheists are evidentialists in one way or another, whether or not they're familiar with the term. And while evidentialism may be
defined as "a theory of justification according to which the justification of a conclusion depends solely on the evidence for it," what
counts as evidence and how to properly infer it is still left open to debate. It is reasonable then, for both the atheist and the theist to explain their methodology for how they infer the ontology of their worldview.
Ontology is the branch of metaphysics dealing with existence. In its most simple definition, ontology is concerned with what there is, what exists, what is real, and what is actual, as opposed to what is merely conceptual or imaginary.[2] But what does it mean to exist? Don't
I exist, and aren't I justified in believing that I do? If I
didn't exist, then who or what would be having the thoughts that I'm having and the experience of writing this blog post right now? Conscious experience can't be an illusion, because the illusion of consciousness
is consciousness. And my experience of the external world that I observe is justified by the fact that I experience it, as my experience itself cannot be an illusion.[3] But when dealing with the external world, one should be less confident of the certainty of its existence considering that we have evidence that the experiences derived from the senses are sourced from a feed of electro-chemical data going from our sense organs to our brain, and it is quite possible that this data can be compromised either in transit or when it's interpreted by the brain. We must always keep open the possibility, however remote, that the external world around us is not real, no matter how uncomfortable this notion may be. Most of us grant as our presuppositions that the external world we experience is for the most part real and that our senses are at least capable of discerning it. This is what philosophers call
basic beliefs. These are the foundational assumptions that virtually all worldviews have to start with to even have a conversation about what exists apart from ourselves. Otherwise, one would have to adopt
solipsism or
radical skepticism, and both of these philosophies are non-starters for discerning what exists apart from ourselves. So without these presuppositions, it is very difficult to make any argument about what exists in the world based on evidence, as any attempt to do so will utterly depend on the presuppositions.
In terms of basic beliefs, I adhere to what I call
epistemological economy. It is similar to the idea of
ontological economy which the philosopher of religion Gregory W. Dawes describes is the view that "we should not posit
new kinds of entities without sufficient reason," or more specifically, "we should not posit a hitherto unknown type of cause without sufficient reason."[4] In
epistemological economy, we should strive for the fewest foundational assumptions possible and we should not posit new kinds of basic beliefs without sufficient reason. The reason why is because the more basic beliefs you assume, the closer you become to assuming your worldview as a presupposition. If, for example, you assume something like a special "sense" that can detect the existence of one particular god as a basic belief, like a
sensus divinitatis, this will
necessarily lead you to one particular god being true, and you could claim to be able to justify the ontology of this god on this basic belief alone. You will be on the path to assuming your conclusion from the start in a manner that either is, or comes dangerously close, to being unfalsifiable. This is little different from
presuppostionalism. Assuming the fewest amount of basic beliefs prevents this.
With that out of the way, the question of
how one infers ontology still remains. There is an array of possible tools we can use for how we decide we are going to best infer the existence of something. The possibilities include (but are not limited to) observation, empiricism, scientific theory and hypothesis, logic, subjective experience, testimony, and faith. How one infers ontology usually depends on the level of importance one places on the epistemologies above, and whether one omits some of them entirely.