Let us begin by considering the notion of change in some more detail As a matter of philosophical speculation, the question of the nature of change has been discussed at least since the pre-Socratics.1 For Heraclitus, for example, change was the permanent state of the world, and for that reason ‘you can never step into the same river twice.’ Others, like Parminides, firmly denied the possibility of change. Aristotle’s contribution to this debate was to introduce the notion of potentiality. Some things are actual, he taught, whereas others are merely potential. Change takes place when something potential is transformed into something actual; when something that could be, but is not, is turned into something that is. For example: a seed is actually a seed, but potentially a tree; a girl is actually a girl, but potentially a woman; a statue of Hermes exists potentially in a chunk of marble. In all cases, change is what turns the one into the other. The world in which we live is the actual world but when previously unrealised potentials are explored and acted on, the actual world changes. Change, in short, is the actualisation of the potential.