In Part I, paper is treated as an H-bond-dominated solid, with an H-bond force constant of <k R>. Application of the elementary principles of statistical mechanics and group theory yields the temperature dependence of Young's modulus directly, without any adjustable parameters. The value agrees quite well with experimental values. A derivation of the effects of water on Young's modulus based on the kinetics of H-bond cooperative break-up is also presented. The maximum value of Young's modulus for an ideal, cellulosic, isotropic paper at maximum density is derived for use in Part II.