Yesterday, I posed (and pointed to an answer for) the question I've used to stop many an economist dead in his/her tracks. But I shouldn't pick on economists (at least not on this count). Most people in most professional concentrations are readily stymied if asked to justify or explain what they do in terms of first principles. Sustainability folk are no different. Ask "what's sustainability for?" or some variant thereon. Then prepare to ignore a certain amount of stammering and nervous weight-shifting.
Oh, we can chirp some now-traditional phrasing about how we're undertaking 'to meet the needs of the present without compromising the ability of future generations to meet their own needs', but most of us haven't thought much about specifically what that might mean. What the implications of that statement are. What contradictions lie under the surface of those words. How that phrasing came to be used, and what (stronger) statements got ditched during the political negotiations required for any international commission to issue a report. As a result, we have little understanding of how and why environmental sustainability became almost synonymous with "sustainability" in general -- relegating social and economic sustainability to secondary status. (For example, check the table of contents of this online text  created by faculty at the University of Illinois. The title is Sustainability: A Comprehensive Foundation, but it's really all about the environment. Kudos to the contributors for the good work they've done, but the word "environmental" really applies far better than "comprehensive" does.)
Everyone with any interest in the nexus of sustainability and higher education should read this excellent (if slightly long) article  from The Hedgehog Review. (You know, "the fox knows many things . . . " (with apologies to the admissions staff at Dickinson College) . . . Gotta love it, for the title alone.) Written by Joshua Yates at UVa, it takes a Cultural Studies-based look at the evolution of sustainability as a concept and a (diverse) undertaking. Good historic summary. Good observations. Good questions that, since we don't have good answers for them at the moment, deserve serious thought and significant discussion.