The mark of true greatness is not to notice that you have received a blow. So does the huge wild beast calmly turn and gaze at barking dogs, so does the wave dash in vain against a mighty cliff. The man who does not get angry stands firm, unshaken by injury; he who gets angry is overthrown.This is from Book II:
Nothing, therefore, is more conducive to anger than the intemperance and intolerance that comes from soft living; the mind ought to be schooled by hardship to feel none but a crushing blow.This second passage (which is earlier in the essay) seems utterly "hardcore," extreme, and macho in a not obviously good way. It seems to reflect what critics of Stoicism would the insensibility of the Stoic ideal. And while I can't countenance all the details of Seneca's writing, I think his arguments against anger, and against the Aristotelian idea that anger can be useful to virtuous action (e.g. as a spur to courage) are worth consideration. For Seneca, anger is essentially a form of temporary madness, and although an emotion it is the product of (always incorrect) judgment. In rejecting anger, Seneca does not argue that we should try to deaden the natural impulses that sometimes result in anger--that is, he distinguishes between anger and our impulsive response to, for example, moral wrongdoing, but argues that this impulse should be governed by reason rather than given over to anger. On the Stoic view, we allow ourselves to become angry--we allow annoyance and frustration to overtake us and, as it were, "overthrow" our reason. This is why actions undertaken in anger will seem, in hindsight, foolish and rash--viz. because they most likely were (and if they were not, we were only lucky that our anger didn't lead us to do something worse).
When we are tempted to anger--when we are affronted by insult or harm--Seneca counsels patience, that is, delaying our response. Importantly, his point is not that we should not respond to those things that often make us angry, but that we delay our response so that it is wise and proportionate. (Often, if we give ourselves time to cool off, we will find, he thinks, that what we received as a harm is not worth the trouble with which we would have repaid it in anger.) This seems like perfectly good advice: Stoic detachment is not an end unto itself, but rather a means to wise (and patient) action. Anger in itself solves nothing and, if Seneca is right, only makes matters--either for ourselves or others--worse.