 More controversially, some philosophers of chemistry have argued that chemical properties may constrain the behavior of physical systems, something akin to what philosophers of mind call strong emergence, or downwards causation (Kim 1999). While acknowledging the central role of quantum mechanics in understanding structure, Hendry argues that in some cases, molecular structure is an unexplained explainer. The issue arises when we consider the quantum-mechanical description of structural isomers, molecules with the same atoms, but with different molecular structures. For example, dimethyl ether and ethanol share a Hamiltonian, the quantum mechanical description of their physical states. Nevertheless, they are very different molecules. Ethanol is extremely soluble in water, whereas dimethyl ether is only partially soluble in water. Ethanol boils at 78.4°C, while dimethyl ether boils at 34.6°C. Drinking ethanol leads to intoxication, while dimethyl ether is toxic in quite different ways. Quantum mechanics can show how each of these structures is energetically stable, and illuminate how they interact with other molecules and radiation to explain the chemical and spectroscopic behaviour of ethanol and dimethyl ether, but the different structures are introduced as unexplained initial conditions. While he acknowledges that these facts are not incompatible with the claim that structure is reducible, Hendry argues that strong emergence is just as plausible an interpretation as reduction of the explanatory relationship between chemistry and quantum mechanics (2006b, 2010a).