These unknown unknowns are by far the most difficult to manage. They are sometimes termed ‘black swan events’ (1) because before black swans were discovered in Australia, no one could imagine the existence of a swan that wasn’t white. Black swan theory was developed by Nassim Taleb to explain:

　　• the huge impact of unpredictable, rare events that are outside our normal experience and without historical precedent

　　• the non-computability of the effect of these rare events because there is no data on which to base calculations

　　• the psychological bias that blinds us to the possibility and impact of rare events. We tend to assume that things (such as property price increases) will continue in a predictable way.

　　The phrase ‘black swan event’ is therefore used as a metaphor for the frailty and limitation of any system of thought and planning: bounded rationality. This means that we cannot know all-important factors that will affect the future (and, anyhow, do not have time to evaluate them). We are, in practice, likely to suffer from bounded rationality even with the known knowns because of imperfect research or pressure of time. However, in a period of turbulence, more events will be in the last two categories and this makes planning more difficult. So how should organisations respond to the threat of unknowns while still trying to move forward in terms of gaining competitive advantage?