unification and explanation: explanation as a prototype concept. a reply to weber and van dyck, gijsberg, and de regt

In this paper I investigate unification as a virtue of explanation. I the first part of the paper (sec. 1-2) I give a brief exposition of the unification account of Schurz and Lambert (1994) and Schurz (1999). I illustrate the advantages of this account in comparison to the older unification accounts of Friedman (1974) and Kitcher (1981). In the second part (sec. 3) I discuss several comments and objections to the Schurz-Lambert account that were raised by Weber and van Dyck (2002), Gijsberg (2007) and de Regt (2005).  In the third and final part (sec. 4), I argue that explanation should be understood as a prototype concept which contains nomic expectability, causality and unification as prototypical virtues of explanations, although none of these virtues provides a sufficient and necessary "defining condition" of explanation.