Duhem? I never even...

Tue 04 Sep 2007 08:25 AM

I am teaching Poincaré and Duhem in seminar this week. They are both so sensible that reading them elicits a twinge of despair at how little progress has been made in philosophy of science since. They were ahead of their time, of course, and there have been some real advances. This is not a post about despair-- or about the bits that were so forward-looking-- so I'll move on.

In Chapter VII of The Aim and Structure of Physical Theory, Duhem claims that the modern dynamical definitions of force and power are not the meanings we get from common sense. We observe that a wagon does not move when there is no horse harnessed to it. With a horse, it moves when the horse pulls. It stops when the horse stops. Thus, Duhem suggests, common sense gives us a quasi-Aristotelean notion of force according to which force is exerted at each moment in which a thing moves. Bodies in motion tend to stop absent exertion.

The example is quaint, since we 21st-century philosophers rarely if ever see horses and wagons. The scepticism about common sense is also different than it would be for us. The 19th-century had been awash with common sense philosophy that was a kind of know-nothingism: a way for educated people to feel comfortable with their dogmas, delivered by common sense in a cellophane wrapper of critical thought.

Nevertheless, Duhem is not entirely dismissive of common sense. He offers a rather nice metaphor of common sense as a trust fund: "The fund of common sense is not a treasure buried in the soil to which no coin can ever come to be added; it is the capital of an enormous and prodigiously active association formed by the union of human minds" (p. 261). As he develops the metaphor, he suggests that anything that is appealed to as an timeless axiom of common sense is rather a withdrawl of a discovery made previously.* If the common sense claim proves unworkable, it may be replaced by a more profitable investment.

That seems right, if we conceive common sense primarily as a source of general dicta like definitions of power-- not the sort of thing we would find perfected in theories that were suited to daily life. Yet Duhem insists that common sense is right in its description of how wagons and horses typically behave. So, he concludes, "observations of common sense are certain to the extent and degree to which they deficient in detail and precision" (p. 264). They suffice as observations but fail as laws.

Since I have an axe to grind, let's distinguish three ways of understanding common sense:

1. Common sense is a reservoir of general principles: that such-and-so is the nature of power, that our senses are reliable, that all things are the work of god, and so on. This is the 19th-century, know-nothing common sense. Duhem advocates falliblism (perhaps even scepticism) about such dogmatic claims.

2. Common sense is a reservoir of particular judgements: that there is a wagon over there, that the wagons we have seen have stopped when the horse rests, and so on. Duhem calls such claims "true and certain", which seems to overstate things.

3. Common sense is a way of forming beliefs: we trust our senses unless their are specific reasons to think we are deceived, we trust our memory, and so on. Duhem does not discuss this explicitly, but it is a natural way to understand the source of the particular judgments that he thinks of as "true and certain."

And the axe that I'm grinding: Thomas Reid, that great source of the common sense tradition, is read too often as advocating 1, sometimes as advocating 2 (eg, by VanCleve), and when read correctly as advocating 3.

* William James argues similarly that what is now common sense was once a discovery. Basic truths, James suggests, were the inventions of genius cave men. The Pragmatism lectures were only a few years after Duhem's discussion appeared as articles, so I wonder if James was familiar with Duhem.