 The second approach to the Indifference Principle focuses on permutation symmetries and proposes to look for those not in a physical system but in the language in use. This approach is due to Carnap (1945, 1955). He maintains, for example, that two sentences ought to be assigned equal prior credences if one differs from the other only by a permutation of the names in use. Although Carnap says little about the Square Case, he has much to say about how his approach to the Indifference Principle helps to justify enumerative induction; see the survey by Fitelson (2006). So objective Bayesianism is often regarded as a substantive account of inductive inference, while many subjective Bayesians often take their view as a quantitative analogue of deductive logic (as presented in  section 4.1).  For refinement of Carnap’s approach, see Maher (2004). The most common worry for Carnap’s approach is that it renders the normative import of the Indifference Principle too sensitive to the choice of a language; for a reply, see J. Williamson (2010: chap. 9). For more criticisms, see Kelly &amp; Glymour (2004).