We like to think we can tell a reasonable belief from a ridiculous one. Yet, Stephen Law, Director of Philosophy at The Department for Continuing Education at the University of Oxford, argues once we're too invested to be wrong, we explain away every inconvenient fact. From conspiracy theorists who believe cats are Martian spies to Young Earth Creationists faced with fossil records - systems of beliefs are self-sealing. Law's answer is vigilance: distrust the beliefs you're most invested in and get comfortable with the gaps rather than fill them in with misguided certainty - as more often than not, they are an intellectual black hole.

I guess most people reading think the beliefs that the moon landings were faked, that fairies are real, and that their dead Auntie is in the room with them are pretty obviously unreasonable. The beliefs that the universe is billions of years old, that Japan exists, and that bacteria cause infections are, on the other hand, considered eminently reasonable. Most of us think we are pretty good at recognizing reasonable and unreasonable beliefs when we come across them. However, we can begin to struggle when it comes to pinning down exactly what makes a belief reasonable or unreasonable.

I am going to outline one tell-tale sign that, while you might think your beliefs are reasonable, they’re not. I call it “But it fits!”

Our beliefs are often challenged. I might believe that the Matterhorn is near Chamonix. Someone points out, using a map, that I am mistaken: the Matterhorn is near Zermatt. I must have been thinking of Mont Blanc. I want to believe what’s true, so, having been given good reason to think I was wrong, I’ll no doubt change my belief.

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However, it can be much more difficult to change our beliefs when we have a great deal invested in those beliefs. Someone who holds religious beliefs may have invested a great deal in those beliefs. They may have donated a large amount of time and money to their church. Losing their religious belief may result in the loss of things that really matter to them—perhaps their sense of who they are, of what’s right and wrong. Loss of religious belief can even result in loss of friends and family, and even career. Someone who has spent their entire professional career advertising cigarettes, believing that cigarettes are fairly safe, has similarly invested a great deal in that belief. The same is true of someone with strong political beliefs, who has spent many years donating to and campaigning for a political movement and whose friends and colleagues all share that same political affiliation. In each, abandoning belief can be very painful, socially, financially, in terms of their sense of identity and purpose. If compelling evidence is presented against such beliefs, it’s unlikely that the person will just change their mind. What’s far more likely is that they will instead adopt the strategy of “But it fits!” in an attempt to ensure that no evidence can count decisively against their belief.

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No matter how much the evidence might pile up against a belief, there’s always a way of making the evidence and the theory fit together.

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Tom believes that cats are hyper-intelligent alien spies from Mars planning an imminent Martian invasion. Tom’s friend Alice tries to convince him he’s being highly unreasonable, and she challenges him with all sorts of evidence: