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The Dappled World: A Study of the Boundaries of Science by Nancy Cartwright

Roger Caldwell introduces us to the untidy but realistic world of philosopher of science Nancy Cartwright.

The picture of the world offered us by the physicists is of one governed by a relatively few, simple, allembracing laws; a world where order and regularity reign. It is a vision that would have been familiar to Scottish philosopher David Hume in the eighteenth century: just as he said, we see regularities in nature and we infer causal connections on the basis of those regularities. The problem Hume saw, of course, was precisely in the nature of that inference. The fact that up to now, a pair of events have always been observed to go together in such a way as to give rise to a description in terms of cause and effect, doesn’t necessarily mean that they will always be so conjoined in the future. What we require is some kind of necessary connection between the two events themselves if we are to be assured that nature will not change tomorrow, and such necessity can never be read off from what are merely regularities.

To assume that the laws of physics are supreme is to remain within the confines of this Humean world. Physical laws assert what are supposedly eternal regularities, but there is nothing necessary about them: we can have no guarantee that the law that holds today will hold tomorrow or, for that matter, that it isn’t just a local effect applying only to our observable part of the universe, and that different laws may apply elsewhere.

For Nancy Cartwright this is all the wrong way about. For her, the world presented by the laws of physics is largely a fiction. The world in which we live, unlike that inside the laboratory, is a messy, unpredictable place, marked by discontinuities and fractures. The regularities promised by physics are rarely apparent. It is a more dangerous – in some ways a more interesting place – than the supposedly absolute and eternal laws of physics would suggest. There is a lack of fit between the laws and reality as we know it: to find the regularities promised you need to look hard and deep and under certain special conditions; you need, ideally, to be in a laboratory.

If not much that happens in nature is, in fact, as orderly and regular as we have been led to believe by physics, then we must expect even less order when we enter the world of the human sciences. Hence, if the economist attempts to lay down laws, he or she is well-advised to equip them with ceteris paribus conditions – that is, if he proposes that “taxes increase prices” he will protect his hide by informing us that they will only do so if other things are equal. But other things rarely are equal. All kinds of countervailing trends may be at work, as well as quite unexpected events – a run on the dollar, an oil bonanza, a devaluation of the currency – so that it is possible that a tax increase, far from raising prices, may be followed by a fall in prices.

Does this mean that the ‘law’ in this case is wrong? Not at all. In explaining why the law failed to apply on this particular occasion the economist will have recourse to counterfactuals: that is, he will explain that the tax increase would have caused a rise in prices if x or y or z had not occurred. In which case, one may think, it is not much of a law, if it cannot guarantee that the cause will give rise to the effect. However, Cartwright argues that this situation is scarcely peculiar to laws of economics; it applies equally to the laws of physics.

Normally the laws of physics do not come to us armed with ceteris paribus clauses: physicists are rather more confident of the robustness of their laws than are economists. For Cartwright, however, this confidence comes from the fact that, unlike economists, physicists are able, in the closed world of the laboratory, to ensure that the outcomes they predict are in fact attained. They create, that is, the severely restricted conditions in which their predictions will come true. Here Cartwright quotes the econometrician Tyrgve Haavelmo who praises the cleverness of physicists who “confine their predictions to the outcomes of their experiments.” When it comes to predicting things in the real world – the world of avalanches, floods, and earthquakes – the task is somewhat trickier.

Take a familiar law of physics such as Coulomb’s law which gives a constant for the repulsion between two negatively charged particles. Not only is it quite easy to find environments in which (due to countervailing factors) that value is different from the predicted one, it is even possible to create environments in which the ‘repulsed’ particles move closer together. This is not to say that the law is wrong, only that instances where it is precisely exemplified are those instances, common in the laboratory but relatively rare outside it, from which all irrelevant factors have been prescinded. The laws of physics operate at a high level of abstraction. It is because of this that they are remote from reality.

This doesn’t entail that our scientific knowledge is put in question. For Cartwright, laws of nature are secondary. What is basic in science, and what we know best, is knowledge of the capacities of things. Laws are simply epiphenomena based on the observed powers that are inherent in nature. Cartwright’s introduction of the notion of capacities (very much akin to the notion of natural powers in the work of Rom Harré) returns causality back to nature. We no longer have the Humean problem of moving from event A to event B by a sort of causal leap of faith, for the invariable association of A and B is not there to begin with.

Rather, the paradigm of scientific knowledge for Cartwright is given by propositions of the form “Aspirins cure headaches”. By this we don’t mean that aspirins always cure headaches – sometimes they do, sometimes they don’t. What we are saying, rather, is that the property of being an aspirin carries with it the capacity to cure headaches. Cartwright requires us to move towards a notion of causality where single predictable outcomes are a rarity. For example, there can be little doubt that a particularly wet weekend is liable to have an effect on the number of road accidents there are likely to be. But the causality works in both directions. The treacherous driving conditions are likely in themselves to result in a higher rate of accidents. On the other hand, the weather may make people drive more carefully; it may even be such as to keep significant numbers at home and off the roads altogether. If the latter then the result may be fewer accidents.

Cartwright, unusually for a philosopher of science, displays a concern not only for science as knowledge but for its potentiality to change the world. If we are practically concerned with science our main interest is less with conceptual purity than with getting things to work. She cites the example of building a superconducting device which would allow us to detect the victims of strokes. Superconductivity occurs when a metal’s electrical resistance vanishes below a certain critical temperature, and is understood in terms of quantum mechanics. In the practical business of designing such a device, Cartwright finds that the supposed primacy of quantum physics disappears. It is the case, rather, that quantum and classical mechanics are applied on an ad hoc basis, as and when they are seen to work. In general she argues that quantum mechanics, rather than offering (as is often alleged) a more basic, ‘truer’ account of the world than classical physics provides, is instead severely limited in its scope of operation. It is not a case of the one being true and the other false. Rather, she argues that quantum physics works only in very specific situations, and not at all when classical physics works best. The latter in no way supervenes on the former. In order to make sense of the world we need both. The world, it seems, does not acknowledge the requirements of scientific faith that it be rational and well-ordered.

Cartwright is in many ways a gadfly in her attack on scientific orthodoxies, and sometimes one might feel that she overstates her case. But if her arguments make the world, and in consequence the science which explains it, messier and more complex than we had supposed, this is a welcome antidote to the simplified notions of science to which we are often exposed by scientists themselves – and, for that matter, by its deconstructionist ‘critics’. Nancy Cartwright’s argument moves always within a realist account of science, though she is careful to distinguish realism from fundamentalism. Her style is forthright and pungent, though she sometimes assumes a greater technical command, say, of econometrics and quantum mechanics than most readers are likely to possess. Nevertheless, any reader daring enough to leap over a few equations will find here some of the most sophisticated and cogent arguments going about the nature of the scientific enterprise.

© Roger Caldwell 2000

Roger Caldwell is a writer who lives in Essex.

The Dappled World: A Study of the Boundaries of Science by Nancy Cartwright is published by Cambridge University Press


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