Derivation of Principle of Extreme Physical Information
  The unknown amplitude law q(x) defining an observed effect may be found using the principle of Extreme Physical Information. EPI is derived as follows. The observations follow an information flow J --> I, with J the information intrinsic to the source and I the Fisher information level in its data, obeying (i) I=4 Integral dx q' -squared. Here q'= dq/dx and p(x) = q(x)-squared is the probability. It was previously shown, using L. Hardy's 5 axioms defining physics, that I = max. Therefore, its variation (ii) delta I = 0. Note that I is generic, obeying (i) for all source effects, whereas J is specific to the particular effect. Hence, rather than having form (i), J obeys (iii) J = Integral dx j[q(x),s(x)] with j some function of its arguments and s(x) a known source, such as of mass, biological fitness, etc. Information I decreases under any irreversible operation such as measurement, so that I l.e. J or, equivalently, I = kJ where 0 l.e. k l.e. 1. Then the variation delta I = k delta J so that property (ii) gives (iv) delta J = 0 as well. Then combining (ii) and (iv), delta(I - J) = 0. Or, I - J = L = extremum. What kind of extremum? Eqs. (i) and (iii) give (v) L=4q'^2 - j[q(x),s(x)]. Differentiating (v), (d^2 L )/(dq'^2) = +8. Then by the Legendre condition the extremum is a minimum. The unknown source effect obeys (vi) I - J= minimum, EPI. 
B. Roy Frieden, Robert A. Gatenby
physics.data-an physics.comp-ph