We present a logical framework and the declarative semantics of a multi-agent system in which each agent can communicate with and update other agents, can update its knowledge state, of its own initiative or when it receives new incoming information, and it is able to prefer beliefs and reactions, when several alternatives are possible. The knowledge state of an agent is represented by an updatable prioritized logic program, in which priorities among rules can be expressed to allow the agent to prefer, and where the preference relation itself can be updated as well. An example is developed to illustrate how our approach works, including how preferring can enhance reactivity in agents. Finally, we discuss some Web applications where our approach can have a significant potential to contribute, and refer to the work of others.