  A recent concept in theoretical physics, motivated in string duality and
M-theory, is the notion that not all quantum theories arise from quantising a
classical system. Also, a given quantum model may possess more than just one
classical limit. In view of these developments, we analyse some general
properties that quantum mechanics must satisfy, if it is not to be formulated
as a quantisation of a given classical mechanics. Instead, our approach to
quantum mechanics is modelled on a statement that is close in spirit to the
equivalence principle of general relativity, thus bearing a strong resemblance
with the equivalence principle of quantum mechanics formulated by
Faraggi-Matone.
