 This is because Stewart began by characterising [SID], ATOT and PCO₂ as independent variables, and [H+] as the dependent variable of interest.He wrote down the equations for equilibrium concentrations derived from the law of mass action, and eliminated all other "dependent" variables.This naturally yielded an equation that phrased [H+] in terms of [SID], ATOT and PCO₂, but people take it as support for the characterisation of variables as dependent and independent.