  The subjective and the objective aspects of probabilities are incorporated in
a simple duality axiom inspired by observer participation in quantum theory.
Transcending the classical notion of probabilities, it is proposed and
demonstrated that all probabilities may be fundamentally quantum mechanical in
the sense that they may all be derived from the corresponding amplitudes. The
classical coin-toss and the quantum double slit interference experiments are
discussed as illustrative prototype examples. Absence of multi-order quantum
interference effects in multiple-slit experiments and the Experimental tests of
complementarity in Wheeler's delayed-choice type experiments are explained
using the involvement of the observer.
