This work examines the derivation of modulus vs. apparent density relationships for ideal, isotropic papers through the use of Percolation Theory. The results of Percolation Theory are linked to the hydrogen bond theory of the structural integrity of paper through the maximum value of Young's modulus of an ideal isotropic cellulosic paper at the density of crystalline cellulose. An activity coefficient provides the linkage between the results of Percolcation Theory and results for real, isotropic papers. The results will be used in Part III To develop relationships between modulus and apparent density for anisotropic, machine-made papers.