Monday, September 14, 2009

Aquinas's design argument and evolution

St. Thomas's Fifth Way is:

We see that things which lack intelligence, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that not fortuitously, but designedly, do they achieve their end. Now whatever lacks intelligence cannot move towards an end, unless it be directed by some being endowed with knowledge and intelligence; as the arrow is shot to its mark by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

A standard question about design arguments is whether they aren't undercut by the availability of evolutionary explanations. Paley's argument is often thought to be. But Aquinas' argument resists this. The reason is that Aquinas' arguments sets itself the task of explaining a phenomenon which evolutionary theory does not attempt to, and indeed which modern science cannot attempt to, explain. In this way, Aquinas' argument differs from Intelligent Design arguments which offer as their explananda features of nature (such as bacterial flagellae) which are in principle within the purview of science.

Aquinas' explanandum is: that non-intelligent beings uniformly act so as to achieve the best result. There are three parts to this explanandum: (a) uniformity (whether of the exceptionless or for-the-most-part variety), (b) purpose ("so as to achieve"), and (c) value ("the best result"). All of these go beyond the competency of science.

The question of why nature is uniform—why things obey regular laws—is clearly one beyond science. (Science posits laws which imply regularity. However, to answer the question of why there is regularity at all, one would need to explain the nature of the laws, a task for philosophy of science, not for science.)

Post-Aristotelian science does not consider purpose and value. In particular, it cannot explain either purpose or value. Evolutionary theory can explain how our ancestors developed eyes, and can explain this in terms of the contribution to fitness from the availabilty of visual information inputs. But in so doing, it does not explains why eyes are for seeing—that question of purpose goes beyond the science, though biologists in practice incautiously do talk of evolutionary "purposes". But these "purposes" are not purposes, as the failure of evolutionary reductions of teleological concepts show (and anyway the reductions themselves are not science, but philosophy of science). And even more clearly, evolutionary science may explain why we have detailed visual information inputs, but it does not explain why we have valuable visual information inputs.


Brandon said...

I don't think it's quite correct to classify the Fifth Way as a design argument, unless by that you means simply a causal argument concluding to the existence of an intelligence. Usually design arguments begin with a characterization of design as a property of the thing being examined; Thomas's argument doesn't. And while the Dominican Fathers translate it as "designedly", the Latin is the much more neutral ex intentione, which contrasts with a casu (translated above by 'fortuitously' -- what happens a casu can only be determined on a case-by-case basis), and arugably is more accurately translated as "from disposition" (what happens ex intentione happens always or for the most part). What Aquinas is really doing is looking at the preconditions for efficient causation -- in an Aristotelian view the final cause is the cause of causes, in that it is what disposes the efficient cause to have this effect rather than that. Purposes in our sense are only one category of Aristotelian final cause.

But I think your argument retains much of its interest in this case; you're right that the argument links uniformity, disposition by final cause, and quod est optimum.

Eric Telfer said...

Nor do evolutionary theories undercut arguments from existence or change; rather, they presuppose such notions, always appealing to secondary agents which change to produce something different, but which had to exist and exist mutably in order to do that, meaning that the first cause type arguments are always more fundamental than evolutionary explanations.

We see the same thing with Big Bang type arguments and explanations, typically.

Also, I agree, as I think Alex would, that the 5th way is not a design argument in the sense of many design arguments, though the term 'design' can, of course, be used in different senses.

James Bejon said...

While I agree that science doesn't really Aquinas's argument, I'm not convinced that science addresses the IDist's argument either. ID, as I understand it, claims that the best explanation of the existence of certain features of the natural world is intelligent design. And I'm not convinced that science really confronts this argument, since I'm not convinced that science (as defined by its practitioners) asks questions like "What's the best explanation for the existence of such-and-such a feature?". Rather, it seems to me that science asks questions like: "What's the best naturalistic explanation for the existence of such-and-such a feature?". Of course, positing a designer needn't be equated with positing the existence of the supernatural, since aliens, for instance, could have seeded planet Earth with life. But this seems irrelevant since science doesn't ask questions like "Is an evolutionary or an alien-seed the best explanation for such-and-such a thing?". So the point, I think, still stands.

Alexander R Pruss said...

I think science asks: (1) "What is the true explanation of feature F?" And, granted, while it assumes that the answer to this question will probably be the same as the answer to the question (2) "What is the best explanation of F?" and that typically this will be the same as the answer to (3) "What is the best naturalistically explanation of F?", it nonetheless offers up the answer to (3) as probably the answer to (1). And by and large it's right to do so.

Anonymous said...

Alex et al,

Speaking about St. Thomas and evolution, has the November conference in Rome (St. Pius V University) been brought to your attention called "The Impossibility of Evolution?"

Abstracts can be found at