Cited in: — Project Management: A Systems Approach to Planning, Scheduling, and Controlling by Harold Kerzner. Google Books. Accessed September 5, 2009. — Checkland, P.B. (1985). Formulating problems in Systems Analysis. In: Miser, H. J. and Quade E. S. (eds.) (1985). Handbook of Systems Analysis: Overview of Uses, Procedures, Applications, and Practice. Chapter 5, pp. 151-170. North-Holland, New York. — Thinking in Systems: A Primer by Donella Meadows. Chapter 1https://en.wikiquote.org/wiki/Poul_Anderson