Space and the Chiral Molecule

According to classical stereochemistry, the molecules of some substances have doubles, in the sense of incongruent mirror-image counterparts. This is the phenomenon of optical isomerism, first identified 150 years ago by Pasteur. In some cases, the double occurs naturally; in others, it has to be artificially synthesized. These molecules thus share a geometrical feature with such familiar objects as our hands, and, indeed, it is this connection that gives the feature its technical name: chirality (from the Greek for hand, kheir). Instances of chirality in chemistry are numerous, especially in living things: examples of chiral molecules include adrenaline, glucose, and DNA. Optical isomerism is interesting, both historically—it played a crucial role in the emergence of structural chemistry and in the attempt to link chemistry with physics— and, I believe, philosophically. I should like to take this opportunity to revisit the scene of an earlier article of mine (Le Poidevin, 1994) in which I examined the implications optical isomerism has for a philosophical debate concerning the nature of space. In that article I argued that chirality in chemistry reinforces a conclusion that Graham Nerlich (1994), in a brilliant reconstruction of a famous argument of Kant’s, had derived from more visible instances of chirality: that we should be realists about the geometrical properties of space. I did not, however, want to follow Nerlich (and Kant) in drawing a more radical conclusion: that we should be realists about the existence of space. That may sound paradoxical, but it is possible (or so I thought) to regard space as a logical construction from its contents and still think of it, qua construction, as possessing certain intrinsic properties that we do not merely impose on it by convention. Since then, I have become more sympathetic to Nerlich’s position. Chirality is best understood by thinking of space as an entity in its own right. So chemistry has some lessons for the philosophy of space. But the pedagogical relation goes the other way, too: the philosophy of space has interesting implications for chemistry.