Coherence : Studies in epistemology and belief revision

Abstract: This thesis consists of seven papers and a comprehensive summary. The subject matter is so-calledcoherence theories of justification, according to which epistemic justification, and ultimately knowledge,is essentially a matter of relations between beliefs, of how well they hang together. The emphasis is onobtaining more precise and accurate versions of some of the most promising theories by working out thedetails of those theories. It is believed that once we have reached a higher level of clarity and accuracy,we will be in a better position to compare coherence theories, with each other and with otherepistemological theories.Two main problem areas are considered: 1) the role of coherence in justification, and 2) coherencetheories of belief revision. Issues concerning holism, the paradoxes of justification and the regressproblem all belong to the first category. Laurence Bonjour's version of holism is discussed and rejectedin favor of a non-holistic (modular) theory. The paradoxes of justification and the regress problem arestudied in the context of Keith Lehrer's coherence theory.The other part of the thesis is devoted to coherence theories of belief revision. One major issue ishow to coherently accommodate new information. The AGM theory is generally regarded as acoherence theory of belief revision but here its coherence interpretation is shown to be problematic.Instead, an alternative theory is advanced based on semi-revision, a kind of revision in which the input isnot always accepted. A second problem is how to improve one's beliefs so that they become coherent (inthe absence of new input). Two main strategies are studied: the subtractive strategy (in which old beliefsare removed) and the addition strategy (in which new beliefs are added). In the case of the additionstrategy, two different models are developed, one of which is based on the idea that the beliefs to beadded must explain the beliefs already present. Representation theorems are provided for all models ofbelief dynamics put forward in this thesis.A connection is established between epistemic justification and belief revision. It turns out that semi-revision (properly generalized) embodies not only an account of coherent revision but also a coherencetheory ofjustification according to which an agent is subjectively justified in believing a proposition justin case that proposition can be coherently embedded into the agent's belief system in a rational way.

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