This paper provides a logical framework of negotiating agents who have capabilities of evaluating and building proposals. Given a proposal, an agent decides whether it is acceptable or not. If the proposal is unacceptable as it is, the agent seeks conditions to accept it. This attitude is captured as a process of making hypotheses by induction. If an agent fails to find a hypothesis, it would concede by giving up some of its current belief. This attitude is characterized using default reasoning. We provide a logical framework of such think-act cycle of an agent, and develop a method for computing proposals using answer set programming.