Armstrong rejects nominalist accounts of properties that attempt to align properties simply with classes. Coextension is a problem they face: if properties are simply classes, in a world where all blue things are also wet, class nominalists are unable to draw a distinction between the property of being blue and being wet. He provides an analogy to the argument in "Euthyphro": to say that electrons are electrons because they are part of the class of electrons puts the cart before the horse. They are part of the class of electrons "because" they "are" electrons.