Comparing Explanations

What makes a strong inference to the best explanation? Two major challenges arise when evaluating the strength of an inference to the best explanation. First, there is the difficulty of determining which candidate explanation is “best.” Second, we face the problem that the true explanation might not be among the candidates we are considering, simply because we haven’t thought of all possible explanations. Let’s consider each of these difficulties in turn.

To decide which explanation is the “best” explanation of some phenomenon, we can assess the candidates in terms of explanatory virtues—characteristics that we believe make an explanation more likely to be true, such as simplicity, breadth, consistency with other things we believe, etc. Scientists frequently employ this approach when considering competing theories or hypotheses. In scientific contexts, explanatory virtues are also called theoretical virtues—characteristics of good scientific theories.

We may draw a subtle distinction between explanatory virtues and theoretical virtues. Not all explanations are theories, and not all theories are explanatory. So, it is possible that some characteristics of good explanations aren’t characteristics of good theories, and vice versa. However, this distinction will make no difference in what follows, since all of the virtues discussed below apply to both theories and explanations. For the sake of simplicity, I will use ‘theoretical virtue’ and ‘explanatory virtue’ synonymously for present purposes.

Theoretical or explanatory virtues include characteristics like the following:

Simplicity: All else being equal, simpler explanations are more likely to be true than unnecessarily complicated ones.
Unification, or breadth: A broader theory—one that unifies or explains more stuff—is better than one that explains less.
Evidential accuracy, or fit: A theory that fits the data closely is better than one that leaves a lot of outliers.

Those are just three examples of theoretical virtues, but there are others as well. In fact, there are at least twelve theoretical virtues—twelve characteristics that make some theories or explanations better (and more likely to be true) than others. There is no definitive, universally accepted list of theoretical or explanatory virtues, even in the sciences. However, historian and philosopher of science Michael Newton Keas has developed what is—to my knowledge—the most comprehensive and systematic treatment of virtues in the literature. Keas organizes twelve theoretical virtues into four classes: evidential, coherential, aesthetic, and diachronic virtues.Keas doesn’t claim that his list is exhaustive: there may be other virtues in addition to these. See Keas (2018), “Systematizing the theoretical virtues,” Synthese 195:2761–2793. This classification schema illuminates the relations between different virtues, as we’ll see below.

It’s not always easy to evaluate the degrees to which competing explanations exhibit each of the explanatory virtues. To complicate the matter, explanatory virtues sometimes conflict with each other. For example, one explanation might be simpler than its rivals, but a slightly more complex explanation might have greater breadth (i.e., it may explain a broader range of phenomena) than the simpler one. In cases like that, we must make the difficult decision which virtue to prioritize: should we favor simplicity over breadth, or the other way around? Moreover, there is no fully objective way of comparing degrees of simplicity, breadth, and so on. For these reasons, philosophers and scientists often must rely—at least to some extent—on their own subjective intuitions and common sense when comparing the virtues of competing explanations.

The second problem is inescapable: we can never be absolutely certain that the true explanation is among the candidates we are considering. The history of science is rife with instances of seemingly “best” explanations that were later rejected when a new, previously unconsidered, explanation was devised. Nonetheless, it is often reasonable to believe the conclusion of an inference to the best explanation with a high degree of confidence, especially when one candidate explanation exhibits many virtues and is clearly superior to its rivals.

For example, the best explanation for my experience of seeing a desk in front of me is surely that there really is a desk here, even if I can’t rule out other possibilities with absolute certainty. I might be dreaming, or perhaps I am living in an illusory world like The Matrix, and maybe there are other possible explanations for my experiences that haven’t even crossed my mind. Even as I acknowledge those other possibilities, the commonsense explanation (that there really is a desk here) is undoubtedly the best explanation for my experience of seeing a desk.

Now that we have a basic understanding of how inference to the best explanation (IBE) works in general, let’s take a closer look at the twelve theoretical/explanatory virtues.