'equilibrium, balance, or harmony of faculties', relates to the 5 spiritual faculties:
wisdom (s. indriya 15-19).
Of these there are two pairs of faculties, in each of which both faculties should well counter-balance each other, namely: faith and wisdom (saddhā, paññā, q.v.) on the one hand and energy and concentration (viriya, samādhi, q.v.) on the other.
For excessive faith with deficient wisdom leads to blind belief, whilst excessive wisdom with deficient faith leads to cunning.
In the same way, great energy with weak concentration leads to restlessness, whilst strong concentration with deficient energy leads to indolence.
Though for both faculties in each of the 2 pairs a balanced degree of intensity is desirable, mindfulness should be allowed to develop to the highest degree of strength. Cf. Vis.M. III- (App.).