 One difficulty for every version of the theory is that functional characterization is holistic. Functionalists hold that mental states are to be characterized in terms of their roles in a psychological theory—be it common sense, scientific, or something in between—but all such theories incorporate information about a large number and variety of mental states. Thus if pain is interdefined with certain highly articulated beliefs and desires, then animals who don't have internal states that play the roles of our articulated beliefs and desires can't share our pains, and humans without the capacity to feel pain can't share certain (or perhaps any) of our beliefs and desires. In addition, differences in the ways people reason, the ways their beliefs are fixed, or the ways their desires affect their beliefs — due either to cultural or individual idiosyncracies — might make it impossible for them to share the same mental states. These are regarded as serious worries for all versions of functionalism (see Stich 1983, Putnam 1988).