Reasoning about trust: A formal logical framework

There is no consensus about the definition of the concept of trust. In this paper formal definitions of different kinds of trust are given in the framework of modal logic. This framework also allows to define a logic for deriving consequences from a set of assumptions about trust. Trust is defined as a mental attitude of an agent with respect to some property held by another agent. These properties are systematically analysed and we propose 6 epistemic properties, 4 deontic properties and 1 dynamic property. In the second part of the paper more flexible notions of trust are introduced: qualitative graded trust, trust defined in terms of topics and conditional trust.