But WP is not as harmless as it might seem, and in combination with U it tightly constrains the possibilities for social choice. This is evident from Sen’s (1970) demonstration that these two conditions conflict with the idea that for each person there is a personal domain of states of affairs, within which his preferences must prevail in case of conflict with others’. This important problem of the “Paretian libertarian” meanwhile has its own extensive literature. For further discussion, see the entry social choice theory.