Gerard de Zeeuw introduced the term ‘Third Phase Science’ in 1997. A deliberative method that is called dialogic design science (see http://dialogicdesignscience.wikispaces.com) illustrates an effective way of implementing third-phase science as a means of understanding and adapting complex social situations. This paper explains De Zeeuw's concept in non-specialist language and expands on the historical context of third-phase science as a means of addressing contemporary needs. It shows how dialogic design science completes third-phase science as an axiomatic science and makes third-phase science into a valuable design methodology. Copyright © 2013 John Wiley & Sons, Ltd.