 To generate epistemological type (i) cases, Kripke asks us to imagine that gold might be blue but only appears yellow thanks to an optical illusion, or that tigers do not normally have four legs, but only appear that way, again thanks to an illusion. However, if the descriptions associated with ‘gold’ were to include ‘is yellow’ and the descriptions associated with ‘tiger’ include ‘is four-legged’ then we would know a priori that these possibilities do not obtain. But that seems to be a mistake. Although we may reasonably be confident that gold really is yellow and not blue, and not just appearing that way, this confidence is not a priori but is instead a consequence of a posteriori experience of the conditions under which we have observed samples of gold. It is a matter of discovery whether we are subject to this kind of illusion, or whether something that is in fact white or grey could be gold; for example it is only a posteriori that we know that gold does not have white and grey allotropes, as tin does. (What is called ‘white gold’ is in fact an alloy of gold; but given a description of its appearance, it is not a priori that it is not gold. This is a contrapositive version of an epistemological type (i) case.) Likewise, turning to epistemological type (ii) cases, if descriptivism were correct, we could rule out a priori the possibility that something has all the superficial properties of gold, but is not gold. But again we do not have such a priori knowledge (again we note that jadeite is superficially similar to nephrite but not the same kind).