To topic of this letter whether it "is better to have moderate emotions, or none at all." Seneca notes two views on the topic of emotions. The Stoics "reject the emotions" and "the Peripatetics keep them in check."
Seneca imagines someone who argues,
it is natural for me to suffer when I am bereaved of a friend; grant some privileges to tears which have the right to flow! It is also natural to be affected by men's opinions and to be cast down when they are unfavourable; so why should you not allow me such an honourable aversion to bad opinion?
While Seneca agrees that emotions do come from Nature in order to "make the indispensable means of existence attractive to our eyes" we should nonetheless acknowledge that "this very interest, when overindulged, becomes a vice."
Vices start with good reason and with some modestly, but then can grow into a vice.
There is no vice which lacks some plea; there is no vice that at the start is not modest and easily entreated; but afterwards the trouble spreads more widely. If you allow it to begin, you cannot make sure of its ceasing.
Therefore, Seneca advocates for cutting them off before they get a foothold.
resist these faults when they are demanding entrance, because, as I have said, it is easier to deny them admittance than to make them depart.
While a wise man can control himself and his emotions, we who are not yet wise should not trust ourselves to think we can control them so easily. He quotes Panaetius:
you and I, who are as yet far removed from wisdom, should not trust ourselves to fall into a state that is disordered, uncontrolled, enslaved to another ... Let us not expose this unstable spirit to the temptations of drink, or beauty, or flattery, or anything that coaxes and allures.
Panaetius was referring to the emotion of love but Seneca observes this advice may be applied to all emotions. He advises strong caution when it comes to dealing with emotions.
let us step back from slippery places; even on dry ground it is hard enough to take a sturdy stand.
Seneca then takes on the claim that the Stoic philosophy is too hard! One may argue,
Your promises are too great, and your counsels too hard. We are mere manikins, unable to deny ourselves everything. We shall sorrow, but not to any great extent; we shall feel desires, but in moderation; we shall give way to anger, but we shall be appeased.
He thinks humans are perfectly capable of keeping motions in check, but instead our love of our vices is so strong we make excuses for ourselves.
And do you know why we have not the power to attain this Stoic ideal? It is because we refuse to believe in our power. Nay, of a surety, there is something else which plays a part: it is because we are in love with our vices; we uphold them and prefer to make excuses for them rather than shake them off.
The reason is unwillingness, the excuse, inability.