Imagine you live in a world free of religious dogma, where science is valued. When someone makes a claim, you assess how plausible the claim is based on what you know to be true. For theories that seem less plausible, you require more evidence.
That is Bayesian reasoning. Someone makes a statement. You assess its plausibility based on all of your experiences up to that point. Think of it as your prior guess as to the likelihood that the statement is true. You then listen to the evidence in favor of the claim, and adjust your beliefs accordingly. That is the posterior likelihood that the statement is true.
If someone is holding an apple and says “if I let go of this apple, it will fly through the air towards the sun,” you would be very skeptical. Before they even let go of the apple, you would guess, based on your current knowledge and prior experiences, that the apple would in fact fall to the ground. You don’t necessarily rule out the possibility that they are correct, you just think it is very unlikely. You would not be confident that they are right until they presented very strong evidence in their favor.
On the other hand, if someone claims “eating apples reduces the risk of cancer,” you might be less skeptical. Maybe you have read about benefits from eating other types of fruits. Maybe you have read something about antioxidants. Your prior guess is that the statement has a reasonable chance of being true. If the person then presents you with results from a randomized clinical trial, you might be quite confident that the statement is correct. Your posterior probability is high. On the other hand, if they presented you with results from a poorly designed survey about nutrition and health, you might not adjust your prior probability much. You would still think the statement might be true, but your confidence hasn’t increased.
Hopefully what I have said so far seems like a reasonable way to assess the plausibility of claims.
Again, suppose you grew up in a world like that.
Now, someone walks up to you and says:
we, through the grace of God the Father, and our Lord Jesus Christ, have seen the plates which contain this record, which is a record of the people of Nephi, and also of the Lamanites, their brethren, and also of the people of Jared, who came from the tower of which hath been spoken; and we also know that they have been translated by the gift and power of God, for His voice hath declared it unto us; wherefore we know of a surety that the work is true. And we also testify that we have seen the engravings which are upon the plates; and they have been shown unto us by the power of God, and not of man. And we declare with words of soberness, that an angel of God came down from heaven, and he brought and laid before our eyes, that we beheld and saw the plates, and the engravings thereon;
You’d probably think they were on an LSD trip. If they went on to tell you about a man named Joseph Smith who was able to read the plates using seer stones, I’d imagine you’d give that a very low prior probability. As Mark Twain put it:
Some people have to have a world of evidence before they can come anywhere in the neighborhood of believing anything; but for me, when a man tells me that he has “seen the engravings which are upon the plates,”and not only that, but an angel was there at the time, and saw him see them, and probably took his receipt for it, I am very far on the road to conviction, no matter whether I ever heard of that man before or not, and even if I do not know the name of the angel, or his nationality either.
Your prior guess would be that Smith is basically a con man. You would need to see strong evidence to the contrary, before you’d take him seriously. And when you looked at the evidence, you’d find out that he made claims that are demonstrably false (such as the origins of native americans). So, you’d adjust your probability downwards, and conclude he was in fact a con man.
Suppose someone told you the following:
Several thousand years ago, God instructed a man named Noah to make an ark for his family and for representatives of the world’s animals and birds. Noah and his family and the animals entered the Ark, and “on the same day all the fountains of the great deep were broken up, and the windows of heaven were opened, and the rain was upon the earth forty days and forty nights”. The flood covered even the highest mountains to a depth of more than 6 metres (20 ft), and all creatures died; only Noah and those with him on the Ark were left alive.
At the end of 150 days the Ark came to rest on the mountains of Ararat. For 150 days again the waters receded, and the hilltops emerged. Noah sent out a raven which “went to and fro until the waters were dried up from the earth”. Next, Noah sent a dove out, but it returned having found nowhere to land. After a further seven days, Noah again sent out the dove, and it returned with an olive leaf in its beak, and he knew that the waters had subsided. Noah waited seven days more and sent out the dove once more, and this time it did not return. Then he and his family and all the animals left the Ark, and Noah made a sacrifice to God, and God resolved that he would never again curse the ground because of man, nor destroy all life on it in this manner. Man in turn was instructed never to eat any animal which had not been drained of its blood.
In order to remember this promise, God put a rainbow in the clouds, saying, “Whenever I bring clouds over the earth and the rainbow appears in the clouds, I will see it and remember the everlasting covenant between God and all living creatures of every kind on the earth”.
Again, you might suspect an LSD trip. Certainly, within seconds, you would list many flaws in the story. You would conclude that the evidence against it is much stronger than the evidence in favor of it.
Suppose someone told you “the earth is about 6000 years old.” You’d take a look at the evidence, that the earth is actually 4.5 billion years old, and conclude that their claim is very unlikely.
Suppose someone told you the story of the Exodus, and the Plagues of Egypt, in particular:
God wanted convince Pharaoh to let the Israelite slaves go. In order to convince him, he (via a man named Moses), turned the water of the Nile into blood (which killed all of the fish), unleashed frogs (via a magic snake staff), gnats, flies, disease, boils, storms, locusts and darkness on Egypt. None of that convinced the Pharaoh. Finally, all Egyptian first born sons were killed, and the Pharaoh relented.
The story sounds unlikely, putting it kindly. It doesn’t make any logical sense. You’ve never seen any magic like that. So, you’d give it a very low probability of being true. Then you look at the archaeological evidence, and find that it is unlikely that the Israelites were even there at the time. So, you conclude that the story is fiction.
Suppose someone told you that God created the first man, named Adam:
When it came time to create Adam, God sent Gabriel, then Michael, to fetch clay from the earth; but the earth complained, saying I take refuge in God from you, if you have come to diminish or deform me, so the angels returned empty-handed. Tabari goes on to state that God responded by sending the Angel of Death, who took clay from all regions, hence providing an explanation for the variety of appearances of the different races of mankind.
After receiving the breath of God, Adam remained a dry body for 40 days, then gradually came to life from the head downwards, sneezing when he had finished coming to life, saying All praise be to God, the Lord of all beings. Having been created, Adam, the first man, is described as having been given dominion over all the lower creatures, which he proceeds to name.
All of this sounds like it could be part of a good fantasy movie. But does it sound plausible? Of course not. You wouldn’t believe it without a lot of evidence. The evidence shows, however, that man evolved from other life forms over hundreds of millions of years. So, you conclude that the story is fiction.
Yet, all of these stories are believed by millions of people, despite the fact that (a) they sound completely absurd and (b) there is little evidence to support the claims, and, in many cases, strong evidence against them.
Hopefully you are at least convinced that these stories would sound absurd to someone who had grown up in a world where evidence-based belief was the norm. I’ll next explore why so many people are convinced that these wild stories actually happened.