  In the Sherrington-Kirkpatrick mean field model for spin glasses, we show
that the quenched average of the free energy can be expressed through a couple
of functional order parameters, in a form very similar to the one found in the
frame of the replica symmetry breaking method. The functional order parameters
are implicitely given in terms of fluctuations of thermodynamic variables.
Under the assumption that the two order parameters can be chosen to be the
same, in the thermodynamic limit, it is shown that the Parisi free energy is a
rigorous upper bound for the free energy of the model.
