  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.
