Yesterday's post brought about the usual accusations that I was Crazy Advisor from Hell, Spawn of Satan, and Meddler Superior into the Lives of Students. But let me put away my pitchfork for a second and step away from the fire on which I am slowly roasting an unsuspecting grad student in order to focus on an interesting question that resurfaced in the comment thread.
There are posts of mine (e.g. here) where a student wants to do something for reasons I may not approve of or understand, or I want to do something for reasons someone else doesn't approve of (e.g. this post). Massimo has commented several times on several of my posts, very consistently, that the problem is always overexplaining. When you offer people a reason why you cannot do something, be ready to have it judged and/or dismissed. So it's best to just say "Sorry, I cannot make/do it" or something to that effect, and leave it at that. There are several other commenters who endorse this approach.
I think Massimo has a valid point, but in my opinion "Sorry, can't do it" is not universally acceptable. Its appropriateness depends strongly on the relationship that the people involved have. Sometimes, "Sorry, can't do it -- period" can do more harm than good as it may seem inappropriately dismissive.
For instance, if a relationship is that of equals, such as between two faculty colleagues, not disclosing details about why you can't meet is fine. But, when the two people have very different positions in a hierarchy and/or one has some power over the other or one is doing the other a favor, then it may not be a good idea for the person lower on the totem pole to appear dismissive, which saying "Sorry, can't do it" without giving a reason does.
Giving a reason indirectly shows that you are considering the proposal seriously and would not really dismiss it without something valid, and as a token of good will you share what it is that you consider important enough to turn down the proposal.
For instance, as in this post, I do not feel it is acceptable to blow off (i.e. stay at a required meeting less than ideal while offering no more than "Sorry, can't do") my program manager, a person I would like to continue to fund my work, without an explanation. I find it less acceptable and more disrespectful than offering an explanation and taking the risk of having its validity judged.
Similarly, I find it more disrespectful/ less acceptable if the student just says "Sorry, can't do" without an explanation to an advisor he'd pestered for months when said extremely-pressed-for-time advisor attempts to schedule a meeting, than it is to offer even a strange excuse.
So what do you say, blogosphere? Is "Sorry, can't do it - period" always acceptable, and, if not, when is it not?