  An assurance case is a structured argument, typically produced by safety
engineers, to communicate confidence that a critical or complex system, such as
an aircraft, will be acceptably safe within its intended context. Assurance
cases often inform third party approval of a system. One emerging proposition
within the trustworthy AI and autonomous systems (AI/AS) research community is
to use assurance cases to instil justified confidence that specific AI/AS will
be ethically acceptable when operational in well-defined contexts. This paper
substantially develops the proposition and makes it concrete. It brings
together the assurance case methodology with a set of ethical principles to
structure a principles-based ethics assurance argument pattern. The principles
are justice, beneficence, non-maleficence, and respect for human autonomy, with
the principle of transparency playing a supporting role. The argument pattern,
shortened to the acronym PRAISE, is described. The objective of the proposed
PRAISE argument pattern is to provide a reusable template for individual ethics
assurance cases, by which engineers, developers, operators, or regulators could
justify, communicate, or challenge a claim about the overall ethical
acceptability of the use of a specific AI/AS in a given socio-technical
context. We apply the pattern to the hypothetical use case of an autonomous
robo-taxi service in a city centre.
