Steel, Daniel P. "Causal Structure and Mechanisms." Across the Boundaries: Extrapolation in Biology and Social Science. : Oxford University Press, 2007-10-01. Oxford Scholarship Online. 2008-01-01. Date Accessed 26 May. 2013 <http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-3>.
Steel, D.(2007-10-01). Causal Structure and Mechanisms. In Across the Boundaries: Extrapolation in Biology and Social Science. : Oxford University Press. Retrieved 26 May. 2013, from http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-3.
Steel, Daniel P. "Causal Structure and Mechanisms." In Across the Boundaries: Extrapolation in Biology and Social Science. : Oxford University Press, 2007-10-01. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-3.
Steel DP. Causal Structure and Mechanisms. In: Steel D. Across the Boundaries: Extrapolation in Biology and Social Science. Oxford University Press; 2007. http://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780195331448.001.0001/acprof-9780195331448-chapter-3. Accessed May 26, 2013.