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Defeaters and Eliminative Argumentation in Assurance 2.0

Robin Bloomfield, Kate Netkachova, John Rushby

TL;DR

This paper addresses how to represent and reason with defeaters in Assurance 2.0 to manage doubts and guard against confirmation bias. It introduces multi-level defeaters and exact defeaters, along with disjunctive/conjunctive decomposition, and anchors these concepts in Clarissa/asce with a logic-programming interpretation via s(CASP). The key contributions are formal propagation rules for three-valued truth, the notion of exact defeaters for eliminative argumentation, and practical tool-supported strategies for recording, refuting, or retaining defeaters to achieve indefeasible confidence. The approach enhances dialectic exploration of arguments, clarifies the distinction between validity and soundness, and provides a path toward scalable, rigorous assurance cases with measurable confidence and metacase transparency for evaluators and developers.

Abstract

A traditional assurance case employs a positive argument in which reasoning steps, grounded on evidence and assumptions, sustain a top claim that has external significance. Human judgement is required to check the evidence, the assumptions, and the narrative justifications for the reasoning steps; if all are assessed good, then the top claim can be accepted. A valid concern about this process is that human judgement is fallible and prone to confirmation bias. The best defense against this concern is vigorous and skeptical debate and discussion in the manner of a dialectic or Socratic dialog. There is merit in recording aspects of this discussion for the benefit of subsequent developers and assessors. Defeaters are a means doing this: they express doubts about aspects of the argument and can be developed into subcases that confirm or refute the doubts, and can record them as documentation to assist future consideration. This report describes how defeaters, and multiple levels of defeaters, should be represented and assessed in Assurance 2.0 and its Clarissa/ASCE tool support. These mechanisms also support eliminative argumentation, which is a contrary approach to assurance, favored by some, that uses a negative argument to refute all reasons why the top claim could be false.

Defeaters and Eliminative Argumentation in Assurance 2.0

TL;DR

This paper addresses how to represent and reason with defeaters in Assurance 2.0 to manage doubts and guard against confirmation bias. It introduces multi-level defeaters and exact defeaters, along with disjunctive/conjunctive decomposition, and anchors these concepts in Clarissa/asce with a logic-programming interpretation via s(CASP). The key contributions are formal propagation rules for three-valued truth, the notion of exact defeaters for eliminative argumentation, and practical tool-supported strategies for recording, refuting, or retaining defeaters to achieve indefeasible confidence. The approach enhances dialectic exploration of arguments, clarifies the distinction between validity and soundness, and provides a path toward scalable, rigorous assurance cases with measurable confidence and metacase transparency for evaluators and developers.

Abstract

A traditional assurance case employs a positive argument in which reasoning steps, grounded on evidence and assumptions, sustain a top claim that has external significance. Human judgement is required to check the evidence, the assumptions, and the narrative justifications for the reasoning steps; if all are assessed good, then the top claim can be accepted. A valid concern about this process is that human judgement is fallible and prone to confirmation bias. The best defense against this concern is vigorous and skeptical debate and discussion in the manner of a dialectic or Socratic dialog. There is merit in recording aspects of this discussion for the benefit of subsequent developers and assessors. Defeaters are a means doing this: they express doubts about aspects of the argument and can be developed into subcases that confirm or refute the doubts, and can record them as documentation to assist future consideration. This report describes how defeaters, and multiple levels of defeaters, should be represented and assessed in Assurance 2.0 and its Clarissa/ASCE tool support. These mechanisms also support eliminative argumentation, which is a contrary approach to assurance, favored by some, that uses a negative argument to refute all reasons why the top claim could be false.
Paper Structure (15 sections, 7 equations, 4 figures)

This paper contains 15 sections, 7 equations, 4 figures.

Table of Contents

  1. Introduction to Defeaters
  2. Representation of Defeaters
  3. Assessing Arguments With Defeaters
  4. Propagation of Positive Assessments
  5. Propagation With Refutations
  6. Exact Defeaters
  7. Summary of Propagation Rules
  8. Logic Programming Interpretation
  9. Comparison with Dialectics in GSN
  10. Eliminative Argumentation and Exact Defeaters
  11. Conjunctive and Disjunctive Decomposition Blocks
  12. An Illustrative Example
  13. Defeaters and Confidence
  14. Summary and Conclusion
  15. Acknowledgments.

Figures (4)

  • Figure 1: Doubt as a Defeater with No Subcase (upper right)
  • Figure 2: Highlighted Defeater Subcase
  • Figure 3: Eliminative Argument Using an Exact Defeater
  • Figure 4: Two Levels of Defeaters: Exploratory Top Right, Exact Bottom Center