Molecular metaphors

‘Philosophy is dead’ is an assertion that, coming from most people, would be dismissed as idle, unconsidered, even meaningless. (What, all of it? Political philosophy? Moral philosophy? The philosophy of music?) But when Stephen Hawking announced this in his recent book with Leonard Mlodinow, The grand design  , it was greeted as the devastating judgement of a sage and sent philosophers scurrying to the discussion boards to defend their subject (more properly, to defend Hawking’s presumed target of metaphysics). 

Yet many chemists may be unaware that a philosophy of chemistry existed in the first place. Isn’t chemistry about practical, tangible matters, or - when theoretical issues are concerned - questions of right and wrong, not the fuzzy and abstract issues popularly associated with philosophy? On the contrary, at least two journals (Hyle and Foundations of Chemistry) and the International Society for the Philosophy of Chemistry have insisted for some years that there are profound chemical questions of a philosophical nature.

These questions might not seem quite as urgent as how to make stereoselective carbon-carbon bonds, but they should at the very least make chemists reflect about the nature of their daily craft. What is the ontological status of ‘laws’ of chemistry? To what extent are molecular structures metaphorical? What’s more, the philosophy of chemistry impinges directly on chemistry’s public image. As Eric Scerri, editor-in-chief of Foundations of Chemistry, says: ‘Most philosophers of science believe that chemistry has been reduced to physics and is therefore of no fundamental interest. They believe that chemistry has no "big ideas" to compare with quantum mechanics and relativity in physics and Darwin’s theory in biology.’¹ 

The philosophy of chemistry excites lively, often impassioned debate. Those unquiet waters have recently been agitated by an extensive overview of the topic published in the Stanford encyclopedia of philosophy, a widely used online reference source, by Michael Weisberg, Paul Needham and Robin Hendry, all three respected philosophers of science.² It’s an ambitious affair, accommodating everything from the evolution since ancient times of theories of matter to the nature of the chemical bond and interpretations of quantum theory. The piece has proved controversial because the authors have presented points of view on several of these issues that are not universally shared. 

Chemical conundrums 

Much of the debate hinges on the fact that the concepts and principles used by chemists - the notion of elements, molecules, bonds, structure, or the idea much debated by these philosophers that ‘water is H₂O’ - lack philosophical rigour. Arguments about whether gaseous helium contains atoms or molecules, or whether the element sodium refers to a grey metal or to atoms with 11 protons, are frequently rehearsed. That these hardly affect the practicalities of chemical synthesis doesn’t detract from their validity as philosophical conundrums. 

Take, for example, Needham’s claim that isotopes of the ‘same’ element should in fact be considered different elements.³ Clearly there is rather little difference between ³⁵Cl and ³⁷Cl, but if ‘element’ is pinned to chemical identity, are H and D really the ‘same’? Does not even the tiniest isotope effect blur any strict definition based on chemical behaviour rather than proton number? Perhaps the Austrian chemist Friedrich Paneth was right to regard the notion of an element as something ‘transcendental’. 

Even more controversially, Hendry takes a view long developed by him and others such as Guy Woolley that the concept of molecular structure is mere metaphor, rendered logically incoherent by quantum mechanics. To distinguish methanol from dimethyl ether, we need to first put the nuclei in position by hand and then apply the Born-Oppenheimer approximation to the quantum equations so that only the electrons move. Without this approximation, the raw Hamiltonian for nuclei and electrons is identical for both isomers. 

Hendry asserts that the isomers exist as quantum superpositions, from which a particular isomer emerges only when the wavefunction is collapsed by observation. Scerri argues,⁴ in contrast, that this collapse happens naturally and inevitably because of environment-induced decoherence. Even if so, the image is disconcerting: molecular structures exist because of their environment, not as intrinsic entities. What of molecules isolated in interstellar space, almost a closed system? Regardless of the position one takes, it remains unclear how, or if, molecular structure can be extracted directly from quantum theory, as opposed to being rationalised post hoc - relative energies can be computed, for sure, but that’s not the same. Ultimately these questions might have answers in physics; at least for the moment, they are philosophical.