Inference to the Best Explanation

When you look at a patch of ground and see regularly spaced shoe-shaped impressions in the pattern that we call “foot-prints”, you immediately infer that someone walked there. What argument are you using? Perhaps this one:

This is a valid deductive argument, but P3 is false. There are other things that could cause such a pattern of impressions in the ground. For example, a machine could easily be made that could do the task. Or they could be produced by a chimpanzee walking upright with shoes on. There may be other ways as well. But even though there are other things that could cause impressions of the relevant sort, it is clear that, in most circumstances at least, the best explanation of the footprints would be that someone walked by. Because of this, it is eminently reasonable to conclude that a person walked by; indeed, unless one had some special evidence to the contrary, it would be irrational not to draw this conclusion. However, the inference taking place is not a deduction; it is an “inference to the best explanation”. If we wanted to lay out the argument it would be as follows:

Inference to the best explanation is constantly used in the sciences, in solving crimes, and in other contexts. In such inferences, one concludes that a certain proposition is true because it would explain a known effect better than any alternative explanation. Recall that an explanation explains an effect by citing causes. An inference to the best explanation concludes that a certain putative cause exists, because it would explain a known effect.

Such an argument depends on knowing (or having excellent reason to believe) several things: (1) that the effect in question exists, (2) that it is an effect (that is, something which is caused by something else), (3) what sorts of things could cause the effect, (4) which of these causes explains the effect best. Notice how these four correspond to P1-P4 in the argument above. We can restate the form of argument involved as follows:

If all the premises are certain, then the degree of support the argument provides for the conclusion is proportionate to how much better the explanation in question is than the alternative explanations available. In the example of the footprints, the alternative explanations are all quite poor, so the conclusion is either certain or nearly so. But in a case where there were several decent explanations, the conclusion would only be probable, or merely possible, depending on how good the other explanations were. For example, think of a murder which any of three people could have committed. The murder is the effect, and there are three explanations corresponding to the three suspects—let’s call them Ed, Fran, and George. Suppose that the best of these three explanations is that Fran did it (perhaps she had a stronger motive than either of the others), but that this explanation was only slightly better. If so, then argument would only make the proposition that Fran committed the murder possible, because though it is more likely that she did it that than Ed did or that George did, it is still more likely that one of the two men did it than it is that she did. Indeed, Fran is probably innocent. So, to assess an inference to the best explanation we need to know not only that the explanation in question is the best one, we need to know how much better it is than competing alternatives.

What makes some explanations better than others in the first place? There are at least three factors: (i) the degree of detail in which the effect is explained; (ii) how much independent reason there is to believe that the cause exists and is operative in the relevant context; and (iii) how well the statement of the cause is circumscribed.

(i). Let’s begin with the first of these factors and consider it in connection with a variant of our footprint example. While on a hike, you come across what we would normally describe as animal tracks. These are the effect that you want to explain. Notice that in describing them as animal tracks, we’re already explaining them as effects of an animal, so for now don’t think of them as animal tracks but as a certain pattern of impressions in the ground. Notice that there are different levels of detail at which this pattern can be described. At the one extreme, they could be described simply as impressions in the ground. A more detailed description would include the approximate size of the impressions and their foot-like shape, and it would indicate pattern in which the impressions occur—for example, it might say that they occur at regular intervals along two roughly parallel lines, and that the impressions are staggered somewhat, so that the impressions in the left line are slightly ahead of those in the right. A still more detailed description would specify the shape, size and pattern more precisely, including such details as whether there are toe marks and how many, the precise shape of each part of each the impression, how deep the impressions are, just how far apart, how exactly each impression is oriented relative to the others, etc. The description could be more or less detailed depending on how many of these aspects of the impression it described and the degree of specificity with which it described each—for example, whether numerical measurements are given, and, if so, with what degree of precision. (There are entire books on documenting animal tracks and some people make this their life’s work.) The upshot of the preceding is that we can describe the effect at different levels of detail. The relevance of this to assessing explanations is that, all other factors being equal, one explanation of an effect is better than another if it can explain the effect in greater detail. So, consider several different explanations that someone might give for the animal tracks: