It says here that farmers have been finding great big circles in their corn fields. Nobody sees how they happen. They've been found in both England and the USA.
Owen: What sort of circles?
Apparently the crop looks as if a large round object has rested on it. Sometimes there are signs of burning. Some people are suggesting that they are caused by UFOs landing.
Charlotte: Yes, I think that's what they could be.
Owen: I don't believe in UFOs. The farmers probably just made the circles themselves.
Well that's possible. But don't forget these reports have come in from different countries. Is it likely that several farmers in different countries suddenly decided to play a joke by reporting circles in their crops? Besides, it's not at all clear how the circles were made.
Charlotte: I think they're caused by UFOs.
The Scottish philosopher David Hume had a way of thinking about this sort of thing. In his day, people were very quick to believe that things they didn't understand were caused by magic, fairies, witches, devils and things. Hume pointed out that when faced with a situation like this one, you need to decide whether it's more likely that (a) circular UFOs have suddenly started landing on the earth, or (b) the reports of circles result from some other cause. Can you think of other possible causes for the reports?
Owen: Easy. The reports might be lies. Maybe there are no circles in the crops.
That's true. Newspaper reporters are sometimes too ready to believe what people tell them. But I have seen pictures of the circles in magazines.
Owen: You might be lying about that.
OK. But let's suppose I'm not.
Owen: Well, hoaxers might have been making circles in the wheat fields.
Yes indeed. People might be doing it as a joke, just to show how gullible others are. Someone might read about the circles appearing in another district, and decide to make some himself. Maybe you just have to get a group of people to run round in a circle, or perhaps they just spray a bit of weed killer in a circle.
What do you think, Charlotte? Is it more likely that aliens have arrived from another planet, or that people are playing a trick?
Charlotte: A trick, I suppose.
What about the people who describe how they were abducted by aliens? Would that be true?
Charlotte: It might be.
OK. But what are the other possibilities?
Owen: The newspapers might be lying. Yes that's true. The newspaper might print a completely false story, just for a joke or to increase their sales. Or the people who say they were abducted might be lying. Anything else?
Owen: Well, the people might really believe they were abducted by aliens.
Very good. How many people are a bit crazy?
Owen: A lot. Maybe one out of every twenty.
Right. So let's look at where we've got to. The newspaper report might be deliberately false; or the person might be lying; or they might be mistaken. When we add up all these possibilities, we see that it's very likely there were no aliens involved. On the other hand, what do you think is the likelihood that the person might really have been abducted by aliens?
Owen: A zillion to one against.