The definition of well being in Sen's capability approach (Sen, 1985, 1993) implies the evaluation of unobservables in a context of complexity and interaction amongst the different capabilities. The issue of measurement of well being in the capability approach is interested by problems related to the difficulties in observing directly the capabilities (a set of opportunities that the individual can convert into observables functionings) behind the achieved functionings and in the very definition of the different dimensions of well being not closed by Sen in a given list. Different techniques have been proposed in the literature to measure well being in the capability approach (see Kuklys, 2005, Robeyns, 2006, Chiappero-Martinetti, 2008, Comim, 2008). Here we aim at showing how, in the field of fuzzy logic, fuzzy expert system can be used to measure well being in the capability approach by focusing on the methods and by referring to its implementation in different areas of the evaluation of well being. The use of fuzzy expert system to measure well being has been proposed in Addabbo, Di Tommaso and Facchinetti (2004) and applied for the evaluation of children well being (Addabbo, Facchinetti and Mastroleo, 2007), of the capability of living an healthy life (Addabbo, Chiarolanza, Fuscaldo, and Pirotti, 2010) while the measurement of the quality of work by using fuzzy expert system has been pursued in Addabbo, Facchinetti, Mastroleo and Solinas (2006). In Section 1 we discuss the mathematical framework of fuzzy logic and the transition from classical logic to fuzzy logic. In Section 2 we present the phases of implementation of fuzzy expert system and in Section 3 we discuss cases of its implementation in the measurement of different areas of well being. Section 4 concludes.
