It irritates me that while I am working on itinearnt ferromagnetism in transition metals, I do not know the origin of the Heisenberg ferromagnetic/aniferromagnetic Hamiltonian, and that I cannot ask my senior this question, because either he will not know the answer, or he will not be able to explain, and further this bit of info is not going to help in research. I cannot ask my guide because he will look upon it as a waste of time, even though he knows the answer. I found a semblance of an explanation in Mattis, which started more or less from first principles, as far as books at this level go. Now if I continue trying to follow it, in the end I may not understand and perhaps the time invested may not turn out to be worth it, 'cos in the mean time there are such a lot of other things to do. But if I leave it unfinished it will keep nagging me for some time.