 An observation that is frequently made about this rule, and that points to a potential problem for it, is that it presupposes the notions of candidate explanation and best explanation, neither of which has a straightforward interpretation. While some still hope that the former can be spelled out in purely logical, or at least purely formal, terms, it is often said that the latter must appeal to the so-called theoretical virtues, like simplicity, generality, and coherence with well-established theories; the best explanation would then be the hypothesis which, on balance, does best with respect to these virtues. (See, for instance, Thagard 1978 and McMullin 1996.) The problem is that none of the said virtues is presently particularly well understood. (Giere, in Callebaut (ed.) 1993 (232), even makes the radical claim that the theoretical virtues lack real content and play no more than a rhetorical role in science. In view of recent formal work both on simplicity and on coherence—for instance, Forster and Sober 1994, Li and Vitanyi 1997, and Sober 2015, on simplicity and Bovens and Hartmann 2003 and Olsson 2005, on coherence—the first part of this claim has become hard to maintain; also, Schupbach and Sprenger (2011) present an account of explanatory goodness directly in probabilistic terms. Psychological evidence casts doubt on the second part of the claim; see, for instance, Lombrozo 2007, on the role of simplicity in people’s assessments of explanatory goodness and Koslowski et al. 2008, on the role of coherence with background knowledge in those assessments.)