Because this definition of chemical hardness shows the same proportionality to polarizability as the Parr-Pearson chemical hardness, the two definitions are indistinguishable on a practical level. On a conceptual level, chemical hardness now shares a common definition in both EQC and CDFT, i.e., as the negative derivative of electronegativity. There is a close analogy in the physical meaning of the two definitions: in both cases hardness represents a resistance of the chemical potential to changes in the number of electrons. 107-109 One fundamental difference is that in CDFT hardness is also the derivative of the chemical potential. As we will show next, analogies between the two frameworks, EQC and CDFT, go beyond the concept of chemical hardness.