  We use the notion of a promise to define local trust between agents
possessing autonomous decision-making. An agent is trustworthy if it is
expected that it will keep a promise. This definition satisfies most
commonplace meanings of trust. Reputation is then an estimation of this
expectation value that is passed on from agent to agent.
  Our definition distinguishes types of trust, for different behaviours, and
decouples the concept of agent reliability from the behaviour on which the
judgement is based. We show, however, that trust is fundamentally heuristic, as
it provides insufficient information for agents to make a rational judgement. A
global trustworthiness, or community trust can be defined by a proportional,
self-consistent voting process, as a weighted eigenvector-centrality function
of the promise theoretical graph.
