Case-based comparative evaluation appears to be an important strat?egy for addressing problems in weak analytic domains, such as the law
and practical ethics. Comparisons to paradigm, hypothetical, or past
cases may help a reasoner make decisions about a current dilemma. W e
are investigating the uses of comparative evaluation in practical ethical
reasoning, and whether recent philosophical models of casuistic rea?somng in medical ethics may contribute to developing models of com?parative evaluation. A good comparative reasoner, we believe, should
be able to integrate abstract knowledge of reasons and principles into
its analysis and still take a problem's context and details adequately
into account. TRUTH-TELLER is a program we have developed that
compares pairs of cases presentmg ethical dilemmas about whether to
tell the truth by marshaling relevant similarities and differences in a
context sensitive manner. The program has a variety of methods for
reasoning about reasons. These include classifying reasons as prin?cipled or altruistic, comparing the strengths of reasons, and qualifying
reasons by participants' roles and the criticality of consequences. W e
describe a knowledge representation and comparative evaluation pro?cess for this domain. In an evaluation of the program, five professional
ethicists scored the program's output for randomly-selected pairs of
cases. The work contributes to context sensitive similarity assessment
and to models of argumentation in weak analytic domains.