To illustrate the theory, Stephen Mumford gives the example of "all ravens are black". Under the theory of Armstrong, Tooley and Dretske, there is a relation of necessity between the universals ravenhood and blackness, rather than there being a relationship with every single raven. This allows the explanation of laws of nature that have not been instantiated. Mumford cites the frequently-used example of the moa bird: "It is supposed that every bird of this now-extinct species died at a young age, though not because of anything in its genetic makeup. Rather, it died mainly because of some virus that just happened to sweep through the population. One bird could have escaped the virus only to be eaten by a predator on the day before its fiftieth birthday." Under the theory of Armstrong, Tooley and Dretske, such a coincidence would not be a law of nature.