Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Sunday, March 20, 2005

Design and explanation

Let's think for a second about explaining how things happen or why.

When we give an explanation of something - philosophers refer to it as the explanandum, the thing to be explained - what is it we are doing? There are a number of accounts in play, but in science, I believe the nomological-deductive (ND) model or "covering law" model and its variants is the best account. Another account is the causal mechanism account in which something is explained if a causal mechanism is offered that is sufficient to cause the explanandum - I have no major objections to this, but I think it can be accommodated in the ND view. A third, as given in the above link, is the "unificationist" account, which is pretty well a "winner takes all" view that owes a lot to evolutionary epistemology, so let's leave it to one side for fear of question-begging.

Under the ND model, explanation is given when something like the following is successfully offered:
M. Model, law or generalisation
C. Initial and boundary conditions
E. Thing (phenomenon or process) to be explained
The "sum" here is a relationship whereby M + C make E likely, or determinate. If M, C and E are all statements representing the states of affairs, then the outcome is to make E true or very likely to be true. It is usually put, since explanation is an act of knowledge, as an inferential relation. Hempel's original version had it as a deductive relationship, but probabilistic and statistical accounts were very soon forthcoming, from him and others.

Okay, enough arcane stuff. Let's consider how we might explain the existence in bats of a membrane between their phalanges that enable them to fly. Evolutionary explanations would run like this:

M. In environment E, if Trait T is fitness-enhancing, T will become fixed or widespread at an equilibrium in the population P
C. [Bat ancestors were in E, had membraneous inter-phalangeal T and T was fitness-enhancing in P]
E. Modern bats have T
Notice something here - the conditions C are not available to us. We do not know what conditions bats were in, and we do not know whether in fact having membranes between the phalanges increased fitness. This is something we infer. In fact, it is something we predict. If we ever find a basal bat fossil, we expect it will have partial membranes and will be, on one account anyway, an arboreal creature, like sugarglider possums and flying squirrels are today.

An explanation allows us to make predictions. If the model M has exceptions, then we are able to refine or extend the model to enhance our knowledge and explanatory ability. This process is called scientific progress. Suppose we find that there are animals that routinely have a fitness-enhancing trait that do not end up at fixation or equilibrium.

Now, how would we explain an E with a design model? Let's take two cases - one where we know that a thing was designed, but not how (the Toledo steel used in the blades), and one in which we know how a thing might be designed, but not that it as. [In the next blog entry, that is.]