 As part of his notorious “modal realism” (see §6 of the entry on  David Lewis),  Lewis identified properties with sets: for example, the property of being a talking donkey is just the set of all talking donkeys (a set which Lewis, surprisingly, held to be nonempty). Few philosophers have accepted this view. The dominant view is that distinct properties can have exactly the same instances, although distinct sets never have exactly the same members. Many are nevertheless happy to apply ‘natural’ to sets (or classes) as well as properties: for example, the class of all green things would be counted as more natural than the class of grue things.