  Argumentation is a promising model for reasoning with uncertain knowledge.
The key concept of acceptability enables to differentiate arguments and
counterarguments: The certainty of a proposition can then be evaluated through
the most acceptable arguments for that proposition. In this paper, we
investigate different complementary points of view: - an acceptability based on
the existence of direct counterarguments, - an acceptability based on the
existence of defenders. Pursuing previous work on preference-based
argumentation principles, we enforce both points of view by taking into account
preference orderings for comparing arguments. Our approach is illustrated in
the context of reasoning with stratified knowldge bases.
