Same evidence, different beliefs

Suppose Ted succumbs to a weird delusion. He believes dogs are spies from the planet Venus. Of course, his friends explain to him that he’s mistaken. They point to the abundant evidence that dogs are benign terrestrial pets, not devious alien plotters. Dogs are clearly not very intelligent, they say. But Ted denies this: they are intelligent, they choose deviously to keep their intelligence hidden. ‘But how do they communicate with Venus?’ ask his friends. ‘They have no radios or transmitters.’ Ted explains again: the transmitters are in the dogs’ brains. ‘But we’ve scanned dogs’ brains and there are no transmitters in there!’ add his friends. Ted responds: The transmitters are made from ingenious material indistinguishable from brain stuff. Each time evidence against what Ted believes is provided, Ted can, with some ingenuity, explain it away. By such means, Ted can make his belief consistent with – fit – the evidence. But if to believe reasonably is to hold beliefs that fit the evidence, then Ted believes reasonably!

Of course, something has gone wrong here. Ted really is utterly deluded. But what’s wrong with his thinking?

For a belief about the world to be reasonable it is not enough that the belief be shown to be consistent with the evidence. As the Ted case shows, any belief, no matter how absurd, can with ingenuity be made consistent with the evidence. That’s not to say it is confirmed by the evidence – actually supported by the evidence.

Many people who believe weird things – such as flat-earthers, 9/11 conspiracy theorists, and young earth creationists – can, like Ted, be highly ingenious when it comes to making their beliefs ‘fit’ the evidence.

Creationists, for example, often explain the fossil record – which very strongly supports the view that the Earth is very old and that species have evolved – by appealing to the narrative of the biblical flood (which drowned all creatures not on the ark and buried them in mud). Just like Ted, creationists construct extremely complex and ingenious explanations for the evidence we observe. Creationist Ken Ham writes:

‘Increasing numbers of scientists are realizing that when you take the Bible as your basis and build your models of science and history upon it, all the evidence from the living animals and plants, the fossils, and the cultures fits. This confirms that the Bible really is the Word of God and can be trusted totally.’ (our italics)

Ken Ham, The Lie: Evolution

Religious people, too, often stress how their ‘worldview’ is just as good a ‘fit’ with what we observe of the world – ‘makes sense’ of what we observe – as is the naturalistic worldview. Again, some religious people claim this establishes that their religious worldview is just as reasonable as the naturalist’s. But, as should now be clear, mere ‘fit’ doesn’t establish that.

So when is a theory actually well-confirmed? That is a contentious philosophical question well worth exploring further, but it’s clear that mere ‘fit’ is not enough.

Question: If more than one theory fits the evidence, how can we choose between them?

The article by Michael Marshall below may help you to consider the question.