The short answer is that there is no fully agreed upon definition of nursing. You could probably argue many different angles. Depending on your instructor, you might do well to support his/her view of it as opposed to taking a more challenging stance.
It's my understanding that the idea of nursing as a profession is what most nursing academics support. Of course, that depends on one's definition of "profession." Often, it seems that nursing is trying to frame itself as equal to, different from, and complementary to medicine. I rarely see arguments for nursing as a profession by comparing nurses to plumbers or hair dressers, though they are also considered professionals under many definitions of the term.
Having a specialized knowledge base is part of many definitions of profession and it seems like that was a major impetus for the development of formalized nursing diagnoses.
Of course, there's no denying that nursing is a practice, especially acute care nursing. There is the application of technical, manual skills and the utilization of previous experiential knowledge. Mathematics, on the other hand, can be purely theoretical (I've heard that anyway). Nursing as a field doesn't exist without patients who need nursing care.
Nursing as work? Seems kinda like a no-brainer - uh, yeah, isn't providing services for money "work" no matter what the service? Nursing does include basic care such as ambulating and feeding. Doing these tasks, I suppose, could be considered *just* work once the patient has been assessed as stable and that's why nurses are able to delegate these tasks to unlicensed personnel.
I personally find this kind of discussion interesting, although consensus is almost never reached.