A view of one's past and other aspects of reasoned change in belief

This dissertation reports work on reasoned change in belief, specifically related to the following concepts: (1) pronominal indexicality in first-order logic (FOL); (2) typicality and range defaults; (3) terminological (language) change over time. Results are presented in each: (i) The pronominal indexical "I"--its meaning changing with who is speaking--is given formal treatment in the context of a logic puzzle, solving a problem previously posed in the literature. (ii) A new form of default information in which typicality is viewed as spreading over a range of possible default conclusions is isolated. "Cardinals are typically red or russet" is a reliable default while both "cardinals are typically red" and "cardinals are typically russet" are not. The range "red or russet" is essential, though shown to require adjustment of previous formalisms. (iii) Terminological change over time, the process of language flexing on the fly as new terms and new meanings become important to a reasoner, is formalized in the context of mistaken past beliefs. The process often is spurred on by contradictory beliefs, which are viewed here as positive aids to reasoned change; a result is proven on recovery from contradictions as well. We concentrate on the latter theme, specifically change in meaning and language usage over time (chapters 1-5); the indexical and default results are presented separately in chapters 6 and 7, respectively. The main technical contributions are in chapters 3-7. In 3 we introduce new concepts for terminological change. In 4, a general theorem about step-wise reasoning in time when contradictions are present is proven. In 5, a step-wise formalism that can handle specific problems of terminological change is presented. In 6, a first-order logic treatment of the first person indexical "I" is given. In 7, some apparently new difficulties in reasoning about typicality are uncovered.