If a property must be present for a thing to belong to a natural kind, that property is a necessary condition for membership in that kind.

For example, in his discussion of gold, Kripke argues that something that is superficially like gold in appearance but lacks the property of having atomic number 79, would not be gold. This establishes that, of necessity, having atomic number 79 is necessary for being gold, but does not show that possession of this property is sufficient for being gold. Similarly, argues Kripke, if we were to discover a population of animals with the appearance of tigers but which turn out to be reptiles, we wou