 Two further problems may be pointed out. First, even molecules with the same number and types of atoms and the same structures may vary in virtue of their spatial orientation (Kalhat 2008). This phenomenon is known as chirality. How can structural properties account for it? Secondly, the composition of structural properties is restricted: not every collection of properties gives rise to a structural property. This violates another principle of classical mereology: the Principle of Unrestricted Composition.