The MECHEM system automates the scientific task of determining the pathway of steps underlying a chemical reaction. MECHEM carries out a systematic search for pathway hypotheses that satisfy constraints and explain the available evidence. During its search, the system conjectures unseen entities as needed to formulate hypotheses, but only after determining by experimentation or other means that all simpler hypotheses are inadequate. The generality of MECHEM's methods is established by pointing out similarities to other scientific tasks and programs.