Fuzzy sets from the ethics of social preferences

We show that the problem of evaluating infinite sequences (or streams) of utilities by a unique utility (or social welfare function) can be stated in terms of fuzzy subsets of the set of infinite utility sequences. For each stream, its evaluation can be viewed as its degree of membership to the subset of ‘ethically acceptable’ streams within the set of possible sequences. Since the property ‘being ethically acceptable’ is not well defined and cannot be exactly measured, the fuzzy approach seems especially adequate.