 This is hardly the last word, however. Having accepted all of the above, there still remains the question of why the logic of measurement outcomes should have the very special form \(L(\mathbf{H})\), and never anything more  general.[7]  This question entertains the idea that the formal structure of quantum mechanics may be uniquely determined by a small number of reasonable assumptions, together perhaps with certain manifest regularities in the observed phenomena. This possibility is already contemplated in von Neumann’s Grundlagen (and also his later work in continuous geometry), but first becomes explicit—and programmatic—in the work of George Mackey [1957, 1963]. Mackey presents a sequence of six axioms, framing a very conservative generalized probability theory, that underwrite the construction of a “logic” of experimental propositions, or, in his terminology, “questions”, having the structure of a sigma-orthomodular partially-ordered set (see Section 4 and the supplement document  The Basic Theory of Ordering Relations  for definitions of these terms). The outstanding problem, for Mackey, was to explain why this poset ought to be isomorphic to \(L(\mathbf{H})\):