 The arguments so far considered show that the superficial properties associated with a kind are, in those cases, not sufficient for kind membership, and so some further property (often a microstructural property) is a necessary condition. Kripke also provides arguments that show that many of the superficial properties typically associated with a kind are not even necessary. Kripke examines Kant’s claim that it is a priori that gold is a yellow metal (Kripke 1980, 118). Let us imagine that the yellow appearance of what we have always taken to be samples of gold is just an optical illusion. Intuitively, we would conclude that we had previously erred in thinking that gold is yellow—we would not conclude that there is no gold. Likewise, one might ask whether animal species are characterized (necessarily) by superficially available qualities, for example, whether a tiger is, by definition, a large carnivorous quadrupedal feline, tawny yellow in colour with blackish transverse stripes and a white belly (Kripke 1980, 119–121). Again, we may consider that these features of the individual animals we have held to be tigers are in fact illusory; that those individuals and others of the same species are, for example, three–legged but that an optical illusion made us think otherwise. This suggests that satisfying a set of readily observable ‘superficial’ properties is not necessary for membership of the tiger kind.