Inference to the best explanation

An inference to the best explanation is when you have some phenomenon or observation that needs explaining, and you conclude that whatever is the best available explanation of that phenomenon is true, just because it is the best explanation of that phenomenon. Sherlock Holmes’ argument at the beginning of this chapter is probably meant to be an inference to the best explanation, although he doesn’t go through all the steps. He concludes, for example, that Watson has taken up medicine again, from the fact that he has a bulge in the side of his top hat of the sort that would be made by carrying a stethoscope around in your top hat (implicit: only practising doctors carry stethoscopes around with them).

To really spell out the details of the argument, he would need to consider some alternative explanations for the bulge in the top hat, and explain why the “Watson has taken up medicine again” explanation is the best of them. But it seems pretty clear that this is what Holmes has in mind – Watson having taken up medicine again is the best explanation of the bulge in his top hat, so the bulge in his top hat is reason to believe that he has taken up medicine again. Detective story reasoning often takes this form. So does scientific reasoning – often the reason we have to believe in some type of unobservable entity (such as electrons) is that their existence is part of a good explanation of observable phenomena (e.g. the fact that the lights come on when you flick the switch).

The argument from design (which is an argument for the existence of God) can be construed as an inference to the best explanation. It looks like this.

Observations:

• organisms are complex and intricate
• they are well adapted to their surroundings
• their parts work together to enable the whole organism to function.

Some possible explanations of the observations:

1. God designed organisms to be just the way they are.
2. Organisms evolved by natural selection – no supernatural forces were involved.
3. Organisms evolved by natural selection, but God designed them to do so.
4. God created organisms 6000 years ago in such a way that it would look as though they had been around much longer and had evolved.
5. Organisms came to be the way they are by completely random processes.

5 is not a very good explanation at all. In considering which explanation of some phenomenon is the best explanation, the first thing to take into account is: If the explanation were true, would the observations be surprising, or would they be just what you would expect? Usually when we’re looking for an explanation, the thing that we’re trying to explain is something surprising – it’s in need of an explanation. What a good explanation does is render it unsurprising. Explanation 5 doesn’t do that – it leaves the complexity and intricacy of organisms surprising.

The other 4 explanations all pass this initial test. Now, to complete the argument, since the conclusion is that God exists, we need reasons for thinking that either 1 or 3 or 4 is a better explanation than 2. There is evidence against 1 (fossils, vestigial organs, etc). However, there is no scientific evidence which would decide between 2, 3 and 4. One reason for preferring 2 might be that it is a simpler explanation. One reason for preferring 3 might be that it explains more: it can account for the origins of life, whereas 2 cannot. (We are not going to settle this question here – we’re just illustrating how inferences to the best explanation work.)

Another example: suppose you observe that, surprisingly, milkmaids don’t get smallpox even at a time when smallpox is rife. What’s the explanation? Milkmaids get the fairly innocuous illness cowpox and having cowpox gives immunity to smallpox.

Now, the fact that milkmaids don’t get smallpox doesn’t conclusively prove that cowpox gives immunity to smallpox. There may be other explanations. Perhaps cows have magical powers that protect those who spend much time with them from getting smallpox. Perhaps milkmaids drink more milk than most people, and milk contains some substance which protects people from smallpox.

What makes those less good explanations, or what would show that those were less good explanations?

• Consistency with other accepted theories. What seems like a good explanation to you will partly depend on your background assumptions. I have the background assumption that magic doesn’t operate in other areas of the world – I don’t need to appeal to magic to explain how I can click on an icon on my computer screen and all your names and student ID numbers come up, for example. You might not share this assumption.
• The results of experimental testing. Do other milk-drinkers have immunity from smallpox? (As it happens, they don’t.)

Inferences from evidence to explanations are not deductively valid. It’s always possible that the explanation is wrong, in spite of the evidence.