  Born's rule is the recipe for calculating probabilities from quantum
mechanical amplitudes. There is no generally accepted derivation of Born's rule
from first principles. In this paper, it is motivated from assumptions that
link the ontological content of a proper physical model to the epistemic
conditions of the experimental context. More precisely, it is assumed that all
knowable distinctions should correspond to distinctions in a proper model. This
principle of "ontological completeness" means, for example, that the
probabilistic treatment of the double slit experiment with and without path
information should differ. Further, it is assumed that the model should rely
only on knowable ontological elements, and that failure to fulfill this
principle of "ontological minimalism" gives rise to wrong predictions.
Consequently, probabilities should be assigned only to observable experimental
outcomes. Also, the method to calculate such probabilities should not rely on
the existence of a precise path of the observed object if this path is not
knowable. A similar principle was promoted by Born, even though he did not
apply it to probability. Another crucial assumption is that the proper rule to
calculate probabilities should be generally valid. It should be applicable in
all experimental contexts, regardless the setup that determines which
attributes of the studied object are observed, together with the probability to
observe each of the associated attribute values. There is no need to refer to
the Hilbert space structure of quantum mechanics in the present treatment.
Rather, some elements of this structure emerge from the analysis.
