A portrait should tell more about the subject than, for example, the way they wore their hair at that time. The portrait should make them immediately familiar as the person they were. It should provide an insight that, because human behaviors are universal, is recognizable even to one who does not know the subject.
If beyond this it has an aesthetic quality that makes it a work of art, then the portraitist has achieved their foremost objective.
-Tracey Archer, 2007