"At the core of his alternative history is the notion of the "adjacent possible", one of those ideas that seems, at first, like common sense, then gradually reveals itself as an entirely new way of looking at almost everything. Coined by the biologist Stuart Kauffman, it refers to the fact that at any given time – in science and technology, but perhaps also in culture and politics – only certain kinds of next steps are feasible. "The history of cultural progress," Johnson writes, "is, almost without exception, a story of one door leading to another door, exploring the palace one room at a time."

Source: http://www.guardian.co.uk/science/2010/oct/19/steven-johnson-good-ideas