Existing approaches to knowledge representation and reasoning in the context of open systems either deal with 'objective' knowledge or with beliefs. In contrast, there has been almost no research on the formal modelling of opinions, i.e., communicatively asserted ostensible beliefs. This is highly surprising, since opinions are in fact the only publicly visible kind of knowledge in open systems, and can neither be reduced to objective knowledge nor to beliefs. In this paper, we propose a formal framework for the representation of dynamic, context-dependent and revisable opinions and ostensible intentions as a sound basis for the external description of agents as obtained from observable communication processes. Potential applications include a natural semantics of communicative acts exchanged between truly autonomous agents, and a fine-grained, statement-level concept of trust.