It's predicated on the reliability (or "truth", if you want to go there) of mathematics, the reliability of the laws of optics and of our optical equipment. And yes, even though there are people who have gone into space and actually orbited the Earth, some might say that even that evidence is predicated on the nature of our phenomenal conscious experience, the reliability of intersubjectivity, and on and on.
When you throw theology into the mix, these concepts can rear their heads pretty quickly:
Atheist: What is the evidence that the Earth is round?
Theologian: We have mathematical evidence, astronomical evidence, and direct observational evidence. It's integral to how satellites work, and even how the GPS on your phone works. The evidence is overwhelming and corroborated by lots of independent disciplines and observations.
Atheist: What is the evidence for evolution?
(Really liberal) Theologian: Evidence from fossils and molecular genetics, which are independent fields of inquiry, give us the same phylogenetic tree. This allows us to predict the distribution of genetic diversity, and observations have been in remarkable agreement with those predictions. We find fossils and gene distributions precisely where we'd expect to. This includes predicting the distribution of latent genes, like the fact that chickens have latent genes for producing teeth.
Atheist: What is the evidence that God exists?
Theologian: Well.... That depends on what you mean by evidence!
Atheist: Well... I suppose I mean the same kind of evidence we've been talking about... empirical evidence that can be independently corroborated, predictions that can be verified or falsified.
Theologian: So you subscribe to verificationism? How do you know verificationism is true – can you verify it? If you insist that we are constrained to empirical evidence for justifying beliefs, you need to justify your evidentialist assumptions.
Atheist: Uh... but... sigh. Okay. Let's say there are other ways of justifying beliefs. What does that have to do with demonstrating the existence of God?
Theologian: You're clearly ignorant of classical foundationalism and reformed epistemology. Perhaps belief in God is properly basic, and needs no a posteriori justification! You shouldn't be commenting on the justification of beliefs until you've read the relevant academic literature.
And down the rabbit hole we go. Broadly speaking, theologians have one of two routes to take when trying to convince others that God, as they happen to define it, exists:
- Natural theology, in which observational evidence about the natural world is used to logically infer the existence of God.
- Claim that evidence is irrelevant
When theists go the other route, which is to argue that there's some "other way of knowing" that God exists – whether it be a sensus divinitus, the anecdotal testimony of others, or some convoluted system of a priori philosophical justification, then the discussion might as well be over. The theist is arguing that they have sufficient justification for believing in God. They may not be able to demonstrate it, they may not be able to actually proffer any evidence, but dammit, they just know. Perhaps it's best summed up by an actual comment from a recent thread on Randal Rauser's blog:
I actually love the idea of being able to believe whatever I want. Since the reality can be literally anything, I'll take my chances and believe in a being that can give us all justice and eternal happiness. It's emotionally appealing as hell, and just prudent.Well, cool. More power to you. Me, I don't like being able to believe whatever I want. I want my beliefs about reality to be contingent on the best available evidence. I want to acknowledge and account for my flawed human memory and my cognitive biases. And I definitely don't want to believe in something just because it makes me feel good.
We don't rehash the fundamentals of our epistemic frameworks every time we have a discussion in which the roundness of the Earth is assumed. We don't have to peel back layer upon layer of epistemic warrant to feel justified in believing something that is corroborated by so many different sources. Why, then, do discussions about God lead down these rabbit trails? I propose it's for a simple reason: If God exists, there should be abundant and uncontroversial evidence; but there isn't any evidence, so theists resort to convoluted theology and philosophy as a smokescreen to shield their beliefs from skeptical inquiry. If you can't justify your belief in God the same way you justify your other beliefs about reality, you have to conjure up some other way to convince yourself it's all true.