To engage in the endeavour to define disability, in a serious and fruitful way, we need first to answer the question why we need to define disability. The answer may be less self-evident than would perhaps be assumed. One could actually argue against the desirability of spending much time and resources on definitions since disability is a source of human variation, which, as such, is enriching society. However, we must also recognise the empirical fact that many members of society are disabled of their ability to participate as full members of society, by their physical or psychological impairments. They might be dependent on special services in order to be engaged in gainful employment, or simply be unable to earn an income and hence be dependent on cash benefits of some kind, mostly paid by the state. What, in the end, is at stake here is the welfare of citizens and how the (welfare) state may enhance and secure the welfare of its citizens by providing various sorts of benefits and services, and designing them properly.