His answer is that an assurance case should make claims about the subject of assurance, and that confidence is communicated by a persuasive argument for those claims. Aristotle identified three modes of persuasion: ethos, pathos, and logos.
Ethos is persuasion by the assertion that the author is qualified (or more generally, competent) to make the claim, which requires demonstration of understanding of the system being assured and the techniques of assurance.
Ethos becomes more persuasive if there is an independent assessment of that understanding. Pathos is persuasion by appeal to the emotions and preconceptions of the receiver.
The resulting confidence can be ill-founded, and the receiver should always be sceptical of the claims and actively seek errors and omissions in the support for the claims.
This support is provided by the logos, or argument, which becomes more persuasive when based on clear and consistent definitions of the terms used, and a systematic breakdown of claims into more focussed supporting claims, until these can be demonstrated by clearly identified evidence.
The seminar illustrated these principles with examples based on the speaker’s long experience, leading to practical advice on areas to consider when commissioning and accepting assurance.
It generated a wide range of questions from the audience that explored these areas in more detail.
The seminar, discussion, and the answers to the questionnaire on experience with assurance cases distributed to the audience will inform the current work of the ISA Working Group on what represents sufficient rigour in safety cases.