 This is not in itself a reason to prefer SI over PI, however. For, PI, just like SI, is an inter-worldly principle of individuation, which means that it, just like SI, could be combined with a Lewisian counterpart theory, thereby preventing swapping yet making room for sliding. It is, in other words, the counterpart theory, and not SI (or PI), which does all the work. In any case, it is not clear that intra-worldly swapping is an empty possibility. According to Ehring, there are circumstances in which a series of slidings constitute one case of swapping, something that he thinks would make swapping more a reason for than against PI (Ehring 2011: 81–85).