 The role of simplicity as a theoretical virtue seems so widespread, fundamental, and implicit that many philosophers, scientists, and theologians have sought a justification for principles such as Occam’s Razor on similarly broad and basic grounds. This rationalist approach is connected to the view that making a priori simplicity assumptions is the only way to get around the underdetermination of theory by data. Until the second half of the 20th Century this was probably the predominant approach to the issue of simplicity. More recently, the rise of empiricism within analytic philosophy led many philosophers to argue disparagingly that a priori justifications keep simplicity in the realm of metaphysics (see Zellner et al. 2001, p.1). Despite its changing fortunes, the rationalist approach to simplicity still has its adherents. For example, Richard Swinburne writes: