Balancing the Formal and Informal in Safety Case Arguments

ABSTRACT. In many safety-critical industries developers and operators are required to construct and present well reasoned arguments that their systems achieve acceptable levels of safety. These arguments (together with supporting evidence) are typically referred to as a “safety case”. Safety arguments historically have been communicated through narrative text, leading often to problems of comprehension and communication. Over the last twenty years there has been increasing interest in using structured argumentation notations such as GSN (Goal Structuring Notation) or CAE (Claims-Argument-Evidence) to communicate the structure of the argument. Whilst such arguments are structured, they remain informal. There is increasing interest in exploring how these informal arguments may be modeled in formal logic, potentially opening up benefits of forms of analysis not possible with informally recorded argument. This position paper discusses a number of considerations in balancing the role of informal and formal logic in modeling assurance case arguments.