 The type-identity theory, if true, might seem to motivate essentialism about psychological kinds, by analogy with other theoretical identities (see Section 1.3.1 and Section 4.3). Kripke (1980), however, denies that his arguments concerning other theoretical identities apply here. Let us say that (slow) pain for us humans is correlated with the firing of C-fibres. If this were a genuine theoretical identity, then in all possible worlds, when someone undergoes C-fibre firing, then they are in a state of pain. But it seems plausible that there could be worlds where C-fibre stimulation is not accompanied by any painful sensation. If there are such worlds, then the theoretical identity implies that such a world is one where people can be in pain without having any painful sensation; indeed the pain might yield a pleasant sensation. Kripke regards this as counterintuitive. The concept of pain is necessarily tied to the phenomenal nature of the pain sensation, whereas the concept of gold is not tied to the colour yellow. Consequently, there is no identity between pain and C-fibre firing (nor any other physical state); likewise there is no physical essence to pain. A posteriori natural kind essentialism fails in this case.