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.