In the days of the hegemony of logic as the main working tool for philosophers of science, the intuition that laws of nature supported counterfactual inferences was very difficult to support. Not only do bodies which fall under gravity in the proximity of a spherical object display a pattern which can be described as ‘v = u + at’, but if the formula is a law of nature, it should support the statement that ‘if a body were to fall (would have fallen) under gravity in the proximity of a spherical object then it would display (or would have displayed) a pattern of motion as described in “v = u + at”’.Suppose the best reasons for believing this formula to be a law are derived from a huge mass of empirical evidence. This evidence must consist of actual cases. How can the scientific community be so bold as to expand the law to cover possible cases that might have occurred in the past, but did not, and which have not yet occurred, but may at some future time? Without this step what sense would it make to send a probe into space which was confidently expected to crash into a comet?In a recent paper, Hiddleston  discusses an example of a singular counterfactual inference that suggests a way of proceeding that could be applied to assess the law-like-ness of chemical equations. He asks us to imagine a situation in which a hiker manages to avoid being killed by a falling boulder by ducking. It certainly seems to make sense to say that had the hiker not ducked he would have been killed. What sustains this inference? The boulder is a powerful particular and maintains its powers whatever the circumstances in which it falls through the forest. The story distinguishes implicitly between the persisting powers of the boulder and the changing circumstances in which they might be exercised, just as the analysis in terms of the concept of natural agency would suggest.Why would anyone be inclined to draw counterfactual inferences from established chemical equations? In practice the point of such inferences is to manage some project, for which the actor, be he or she lay or professional, imagines the circumstances. It might be an experiment. It might be the design of an industrial plant to manufacture a certain compound on a large scale, say dioxin. Philosophers have discussed inferences to imagined circumstances in terms of the metaphor of ‘possible worlds’. The force of a counterfactual inference differs depending on the ‘distance’ of the imagined circumstances, or possible world, from the actual world. What could persist through differing circumstances? The boulder example suggests that it must be powerful particulars. In chemistry, while the discussion remains in the context of the homogeneous chemical regress of acids, bases, elements and compounds, the relevant particulars must be samples of chemical substances. But they are not agentive in the context of scientific chemistry.If we update the basis of chemistry to include the heterogeneous regress of charges, Coulomb forces between ions, electron transfer and so on, once again a repertoire of powerful particulars is available to sustain counterfactual inferences. The intuition that samples of ammonia and hydrochloric acid would yield ammonium chloride if they were collocated is sustained by the continuity in space and time of the ions that electron exchange has created. 3 The equation 3When I was a child my father demonstrated this reaction by holding a stick soaked in hydrochloric acid over the midden behind the cowshed on our farm, when a white cloud of ammonium chloride was produced.
sustains a counterfactual just because we believe in the integrity of ions within a certain range of circumstances in which they are active. Were ammonia to react with hydrochloric acid ammonium chloride would be formed. Some situations are incompatible with the persistence of ionic activity, and so are not appropriate for counterfactual inference. Designing an industrial plant that way will be a commercial failure.
Despite the lack of ‘legal’ nomenclature the literature of chemistry is full of statements expressing law-like propositions. That this is the proper status to which they should be assigned is evident in the fact that they are about a model world of nicely behaving atomic entities and forces that resembles the real world quite closely, so far as we can tell. Furthermore their scope is general within their ranges of convenience, and, supported by hypotheses about the relevant causal mechanisms, they are naturally necessary. However, chemical equations have this status only by virtue of the fact that they lie in the ground level of a homogeneous regress of chemical concepts, and in the highest level of the second heterogeneous regress on which the rationality and so the necessity of chemical rules depends.
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