 Sentences are, in a sense, capable of expressing distinctions that propositions cannot. For example, the two sentences \(p\) and \(\neg \neg p\) are obviously distinct. But if \(p\) and \(q\) are provably equivalent, i.e. if \(\vdash p \leftrightarrow q,\) then \(\{p\}\vdash q\) and \(\{q\} \vdash p.\) By Soundness, \(\{p\} \vDash q\) and \(\{q\}\vDash p.\) Therefore, for any valuation function, \(V(p)=V(q).\) So \(p\) and \(q\) must express the same proposition. Of course, an agent who is unaware of the equivalence might believe \(p\) without believing \(q\). What's worse, every sentence \(p\) such that \(\vdash p\) must express the tautological proposition \(W\). Of course, ordinary agents do not always recognize theorems of propositional logic. For this reason, some argue that it is sentences, rather than propositions, that are the appropriate objects of belief. However, most of the proposed models we will study require that rational agents adopt the same belief attitude toward logically equivalent sentences. That is a very strict requirement, amounting to an assumption that every rational agents is logically omniscient, i.e. she finds all logical entailments to be completely transparent. So long as that is the case, there is no significant difference between taking the objects of belief to be sentences or propositions. See, however, Hacking (1967), Garber (1983) and Pettigrew (forthcoming) for ideas on how to relax the requirement of logical omniscience. Still others are not satisfied with either sentences, or propositions. Perry (1979), Lewis (1979) and Stalnaker (1981) argue that in order to capture  essentially indexical beliefs–beliefs that essentially involve indexicals such as  I, here, or  now–the objects of belief must be  centered propositions. We will not take up this helpful suggestion here, but see Ninan (2019) or Liao (2012) for more on centered propositions.