High quality thoughtfulness applied to conceptual matters.
NOT the "letter of the law" approach to thinking.
If you're arguing in court, or fighting someone in a debate, what's important is to WIN at any price! The quality of philosophy, and such things as fairness, suffer. But instead . . .
The systems of REASONABLENESS utilise:
Simultaneous multiple-viewpoint tracking.
Floating-level consideration of philosophical ideas.
Wide leeway in accommodation of different viewpoints and interpretations.
Thinking along with other ways of thinking and trying to find compatibilities rather than faults.
Furtherance of philosophical science and the Truth being more important than winning an argument.
Another aspect of the Reasonableness principle is to have flagged that which has UNREASONABLENESS. If you can spot a mode of argument using various unfair methods and tricks, the motives of that party can be revealed to be unworthy of good science.
Minor point: Unreasonableness is sometimes confessed by some authoritarianists by the use of the term "Zero Tolerance".