From the 3rd book of the dialogues of Seneca to Novatus:
May it not be that, although anger be not natural, it may be right to adopt it, because it often proves useful? It rouses the spirit and excites it; and courage does nothing grand in war without it, unless its flame be supplied from this source; this is the goad which stirs up bold men and sends them to encounter perils. Some therefore consider it to be best to control anger, not to banish it utterly, but to cut off its extravagances, and force it to keep within useful bounds, so as to retain that part of it without which action will become languid and all strength and activity of mind will die away.
In the first place, it is easier to banish dangerous passions than to rule them; it is easier not to admit them than to keep them in order when admitted; for when they have established themselves in possession of the mind they are more powerful than the lawful ruler, and will in no wise permit themselves to be weakened or abridged. In the next place, Reason herself, who holds the reins, is only strong while she remains apart from the passions; if she mixes and befouls herself with them she becomes no longer able to restrain those whom she might once have cleared out of her path; for the mind, when once excited and shaken up, goes whither the passions drive it.
There are certain things whose beginnings lie in our own power, but which, when developed, drag us along by their own force and leave us no retreat. Those who have flung themselves over a precipice have no control over their movements, nor can they stop or slacken their pace when once started, for their own headlong and irremediable rashness has left no room for either reflexion or remorse, and they cannot help going to lengths which they might have avoided. So, also, the mind, when it has abandoned itself to anger, love, or any other passion, is unable to check itself: its own weight and the downward tendency of vices must needs carry the man off and hurl him into the lowest depth.