Biological membranes selectivity for solutes is measured by the reflection coefficient σ, defined as σ = (ΔP/Δπ)Jv=0 where ΔP is the hydrostatic pressure difference required to zero the volumetric flow Jv driven by an osmotic pressure difference Δπ of the considered solute. It as been shown by means of non-equilibrium thermodynamics that σ = (-LPD/LP) where LP and LPD are filtration and ultrafiltration coefficients. Now, if the permeating nonelectrolyte is also actively transported by the membrane, volumetric flow is given by Jv = LPΔP + LPDΔπ + LPAYA where YA is a force sustaining active transport and LPA the phenomenological coefficient, coupling that force to volumetric flow. In Kedem and Katchalsky treatment YA is the affinity of the chemical reaction sustaining active flux, while Diamond proposed the introduction of an active transport potential with the characteristics of a chemical potential. Other proposals can be made, but the definition of YA is not important for subsequent treatment.