Futures research, foresight and speculative design often use the notion of "possible", "potential", "plausible" and "preferable" future. See for instance the work by Stuart Candy who uses the diagram above based on the work of different researchers. This typology is described in more details in these two articles:

Hancock, T. & Bezold, C. (1994). Possible futures, preferable futures. Healthcare Forum Journal, vol. 37, no. 2, 23–29

Voros, J. (2001). A Primer on Futures Studies, Foresight and the Use of Scenarios, prospect, the Foresight Bulletin, No 6, Swinburne University of Technology

Why do I blog this? It seems that this graph spread like an internet meme, and it's relevant to get back to how it has been produced, why and how.