Underdetermination and Rational Choice of Theories Jacob Busch Received: 29 November 2007 / Revised: 24 February 2008 / Accepted: 9 April 2008 / Published online: 8 May 2008 # Springer Science + Business Media B.V. 2008 Abstract The underdetermination of theory by data argument (UD) is traditionally construed as an argument that tells us that we ought to favour an anti-realist position over a realist position. I argue that when UD is constructed as an argument saying that theory choice is to proceed between theories that are empirically equivalent and adequate to the phenomena up until now, the argument will not favour constructive empiricism over realism. A constructive empiricist cannot account for why scientists are reasonable in expecting one theory to be empirically adequate rather than another, given the criteria he suggests for theory choice. Keywords Theory choice . Constructive empiricism . Underdetermination of theory by data . Inference to the best explanation The Challenge Posed by Underdetermination of Theory by Data Argument Scientific realism (SR) is often characterised as the view that scientific theories are truth-apt (semantic SR), that they aim at truth (axiological SR) and that they describe a mind-independent reality (metaphysical SR). Moreover, scientific realists hold that we are rationally warranted in believing that our best scientific theories are true (epistemic SR). Arguably one of the strongest arguments against scientific realism is the underdetermination of theory by data argument (UD). UD comes in varying strengths, but I take it to have the following basic formulation: 1) At any time, any scientific theory has empirically equivalent competitors. 2) If theories are empirically equivalent, then there is equally good reason to believe that either is true. Philosophia (2009) 37:55–65 DOI 10.1007/s11406-008-9133-9 J. Busch (*) Department of Philosophy, University of St Andrews, Edgecliffe, The Scores, St Andrews KY16 9QS, UK e-mail: jb30@st-andrews.ac.uk