Catholic Encyclopedia of 1913: Article on Virtue, especially note segment on “Mean of Virtues” under “Properties of Virtues”
One of the properties of virtues is that they consist in the golden mean, that is to say, in what lies between excess and deficit. For as the perfection of things subject to rule consists in conformity with that rule, so also evil in those same things results from deviation from that rule either by excess or defect. Hence the perfection of the moral virtues consists in rendering the movements of the appetitive powers conformable to their proper rule, which is reason, neither going beyond nor falling short of it. Thus fortitude, which makes one brave to meet dangers, avoids on the one hand reckless daring and on the other undue timidity.