01
The idea
Aristotle's position, which Seneca is arguing against throughout, is that anger has a proper role: measured, at the right people, for the right reasons, in the right degree. On that view anger is an energy source for justice, and its total absence is a defect.
Seneca's counter is not that anger is too strong. It is that anger will not accept terms. Admit it as a servant on condition that it stay proportionate, and it will not honour the condition — because the passion, once assented to, sets its own limits.
His constructive claim is that everything anger is credited with can be done better without it. The severe punishment, the firm correction, the resistance to injustice — all available to a resolved person, and available in a form that stops when it should.
02
Why this matters
This is the argument that decides whether Stoicism has anything to say to you about injustice. If anger is required to resist wrong, then a philosophy of composure is politically useless. If it is not, then Stoic resolve is the better instrument.
The practical stakes are personal too. Most people carry a belief that their anger is what makes them take action, and therefore protect it. Testing that belief is worth doing, because it is often false — the anger produced the outburst and something colder produced the change.
And the honest presentation of both sides is itself the lesson. A department that teaches you to examine evidence cannot present a two-thousand-year-old live dispute as settled.