The following is an inexact transcript of a conversation that happened exactly like this. The scene: a wine bar in Manhattan, on my second (and final) date with a Jewish girl. We’ll call her “Jewish Girl on Date”, or J-GoD for short.
J-GoD: You’ve changed your OKCupid religion status from “Jewish” to “atheist” since last week. What happened this weekend that proved to you that God doesn’t exist?
Jacob’s inner voice: Actually, I switched it to optimize my dating profile and avoid Jewish girls that give me grief about not being as Jewish as their moms expect me to be.
Jacob’s mouth: I don’t think that anything can really prove that God doesn’t exist. That’s partly because the definition of God will usually shift to accommodate any evidence.
J-GoD: So why do you call yourself an atheist if you can’t prove that God doesn’t exist?
Jacob: I give the existence of any specific god a low enough probability that I functionally behave as if I was sure no god existed.
Jacob: I give about a 1 in 10 chance for the existence of any popularly conceived supernatural beings, including humanity’s descendants simulating our reality. For some specific religion’s god, like the Old Testament Jewish God (we’ll call him J-God for short), something like 1 in 1,000,000.
J-GoD: How can you put a number on the existence of J-God?
Jacob: Umm, I have this blog about how you can put a number on almost anything… Anyway, probability numbers are how I represent how confident I am that something is true or not.
J-GoD: How the hell can you be exactly one in a million confident that God exists?
Jacob: I wish I could say that I calculated the prior of the Kolmogorov complexity implied by the description of J-God and updated on all available evidence. In reality, I just picked a really low number that matches how confident I allow myself to be on complex metaphysical questions.
J-GoD: So you’re just making up a number to say that you think that God doesn’t exist?
Jacob: No, no, the exact number is important. For example, if I was walking down the street and suddenly saw a bush burst in flames, and the bush burned but wasn’t consumed, and I heard a voice from the sky saying: “I am the God of your father, God of Abraham of Isaac and of Jacob“, I would definitely update my belief.
It’s possible that I could see a divine bush in a godless world as the result of hallucinogenic drugs or a convoluted prank involving VR, but I’m much more likely to see it in a universe in which J-God exists. In J-God’s universe pranksters and drugs still exist, but so does a divinity that is known for using burning bushes to impress people. Let’s say that a burning bush is one hundred times more likely in a J-God universe. So, I would update my belief in J-God by a factor of one hundred, from 1 in 1,000,000 to 1 in 10,000. That’s a high enough probability of J-God watching over me that I would at least make sure to never again boil a goat in its mother’s milk.
A second miracle would bring my posterior belief in J-God from 1/10,000 to 1/100, far above any other single supernatural being and high enough to give some real bite to Pascal’s wager. At three independently observed miracles, I will switch to living a life of humble devotion to J-God.
J-GoD: You think that people should only believe in a God after they see him perform exactly three miracles? That’s a perverse notion of belief! Belief in God has nothing to do with seeing miracles!
You know how widespread were in those days the opinions of the Sabeans: all men, except a few individuals, were idolaters, that is to say, they believed in spirits, in man’s power to direct the influences of the heavenly bodies, and in the effect of talismans. Any one who in those days laid claim to authority, based it either [on reasoning and proof] or that some spiritual power was conferred upon him by a star, by an angel, or by a similar agency.
He basically says that for people who see magic in every charlatan and miracles every other Tuesday, a miracle should not constitute strong evidence. This is sound Bayesian reasoning. However, we are no longer “in those days”. As an educated rationalist in 2016, I don’t believe that supernatural wonders are common at all. Seeing a true miracle with my own eyes would provide solid grounds for changing my belief.
In Mishne Torah, Maimonides agrees that the performance of miracles should at least make you consider that you’re dealing with a genuine, Twitter-verified, message from the divine, i.e. a prophet:
Just as we are commanded to render a [legal] judgment based on the testimony of two witnesses, even though we do not know if they are testifying truthfully or falsely, similarly, it is a mitzvah to listen to this prophet even though we do not know whether the wonder is true or performed by magic or sorcery.
By “magic and sorcery” Maimonides means illusions and tricks, as opposed to true divine intervention. For example, hallucinogenic drugs and VR count as “magic and sorcery”. Now of course, Maimonides knows that 0 and 1 aren’t probabilities, so Bayesian updating on evidence cannot bring a man to absolute and total belief. As long as drugs or VR are a possibility, they cannot be completely discounted as the source of the observed miracle.
From Mishne Torah again:
The Jews did not believe in Moses, our teacher, because of the wonders that he performed. Whenever anyone’s belief is based on wonders, the commitment of his heart has shortcomings, because it is possible to perform a wonder through magic or sorcery.
Here’s a great (atheist) Jew explaining how a great (deeply religious) Jew proved that two smart Jews shouldn’t disagree on their picture of reality. Maimonides and I don’t have the shared knowledge required to reach consensus, but we are in complete agreement regarding the proper epistemology of miracle-based belief in J-God.
We differ in our moral value judgment on less-than-absolute belief: I believe that it is a virtue, Maimonides that it is a shortcoming. However, I am a moral anti-realist: I believe that moral value judgments are a fact about (my and Maimonides’) minds, not about external reality. Thus, our moral disagreement isn’t cause for concern for me that I am irrational on the subject.
J-GoD: What kind of atheist are you that you analyze in minute detail the biblical commentary of medieval rabbis?
Jacob: What kind of Jew would I be if I didn’t?