Rensberry.com
Reasonable people think about their needs, and the needs of others, and adjust their behavior to meet the goals of a common good for the community, i.e., expressing what you want to say, but accepting and accommodating the needs of others.
Mary Shaw's explanation:
The Reasonable Person Principle is part of the unwritten culture of CMU computer science. It holds that reasonable people strike a suitable balance between their own immediate desires and the good of the community at large.
As applied to bulletin boards, this would include things like observing the explicit or implicit ground rules about subject matter or tone. These vary from one bulletin board to another but usually include sticking to the expected subject matter and refraining from personal attacks. There are exceptions, though. For example, cs.opinion is no-holds barred and often both aggressive and personal.
Not all people share the same model of reasonableness, so disagreements inevitably occur. Under the reasonable person principle, the first thing to do is work it out privately (perhaps in person, since e-mail is known to amplify feelings). Indeed, many people would find it unreasonable to bring in third parties before trying personal discussion.
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