Abstract: A method for defeasible reasoning including a method for analyzing data input, reasoning to conclusions on the basis of that data and adopting and executing plans on the basis of those conclusions. The processes are defeasible in the sense that, as reasoning proceeds or new data is input to the system, both conclusions and plans may be retracted and replaced by new conclusions and plans that are better supported by the new data and reasoning. The methods of the invention are useful as controlling mechanisms for a variety of applications, including robotics, operational assembly lines, and as on-line "assistants" for computer work. The methods can also be used as an advisor in many applications, including medical diagnosis, fault diagnosis in manufacturing and transportation scheduling. Additionally, submethods that draw conclusions on the basis of defeasible reasoning can be used as an inference-engine for automated data analysis.