Should this power depend on strength of understanding alone?
We doubt it.

The fact that there are men of the greatest intellect who cannot command themselves certainly proves nothing to the contrary, for we might say that it perhaps requires an understanding of a powerful rather than of a comprehensive nature; but we believe we shall be nearer the truth if we assume that the power of submitting oneself to the control of the understanding, even in moments of the most violent excitement of the feelings, that power which we call SELF-COMMAND, has its root in the heart itself.

It is, in point of fact, another feeling, which in strong minds balances the excited passions without destroying them; and it is only through this equilibrium that the mastery of the understanding is secured.

This counterpoise is nothing but a sense of the dignity of man, that noblest pride, that deeply-seated desire of the soul always to act as a being endued with understanding and reason.

We may therefore say that a strong mind is one which does not lose its balance even under the most violent excitement.
If we cast a glance at the variety to be observed in the human character in respect to feeling, we find, first, some people who have very little excitability, who are called phlegmatic or indolent.
Secondly, some very excitable, but whose feelings still never overstep certain limits, and who are therefore known as men full of feeling, but sober-minded.
Thirdly, those who are very easily roused, whose feelings blaze up quickly and violently like gunpowder, but do not last.
Fourthly, and lastly, those who cannot be moved by slight causes, and who generally are not to be roused suddenly, but only gradually; but whose feelings become very powerful and are much more lasting.