Science is essentially anarchistic: theoretical anarchy is more humanitarian and likely to encourage progress than doctrines based on law and order (Feyerabend 1979). In view of this and encouraged by Van de Ven, I have looked not for one but for several explanatory theories borrowed from related fields. I consider these theories to be ‘useful’ in the sense given by Penrose (1989), as quoted by Bygrave and Hofer (1991: 16):