The letter begins with a question.
"Whom then of the gods," you ask, "have you found as your voucher?"
The footnote of this letter provides a bit more context with regard to the term 'voucher'. It states:
One who incurs liability by taking upon himself the debt of another. It is part of the process known as intercessio.
Having been raised Christian, I immediately recognized the concept of someone taking on the sins (debts) of another. In Christian theology, the one to takes on the sins of others is Christ - some would call him god.
Seneca explains that the Stoic voucher - or god - is
A god ... who deceives no one, – a soul in love with that which is upright and good.
Men deceive, but god does not. God has no need of deception, but men, who want to manipulate and control others, deceive.
Stoic philosophy would have us steer clear of luxury and decadence. Seneca states as much.
Proceed as you have begun, and settle yourself in this way of living, not luxuriously, but calmly. I prefer to be in trouble rather than in luxury; and you had better interpret the term "in trouble" as popular usage is wont to interpret it: living a "hard," "rough," "toilsome" life.
But ought we to pursue a life full of slavery, toil and dread? Neither luxury nor drudgery are the wise course of action. We are to prepare for toughening and when Fortune sends trials our way, we are prepared to act virtuously toward them. But as for luxury and decadence - shun them.
If you do have free time, then study philosophy in order to more perfectly practice it.
gird yourself about with philosophy, an impregnable wall. Though it be assaulted by many engines, Fortune can find no passage into it. The soul stands on unassailable ground, if it has abandoned external things; it is independent in its own fortress; and every weapon that is hurled falls short of the mark.
Let us then recoil from her as far as we are able. This will be possible for us only through knowledge of self and of the world of Nature.
Note above, the reference to the three disciplines: knowledge of self, as related to the discipline of assent; knowledge of the world, as related to the discipline of action; and knowledge of Nature, as related to the discipline of desire.
The ultimate preparation and toughening is the preparation for death.
strength of heart, however, will come from constant study, provided that you practise, not with the tongue but with the soul, and provided that you prepare yourself to meet death.
Learn philosophy, then practice it. Practice it by contemplating death so as to not fear it at all.
Death falls under the category of indifferent. And just like all other indifferents, we are to use them in order to exercise our excellence. For we all die, but to die with excellence is 'up to us.'
I classify as "indifferent," – that is, neither good nor evil – sickness, pain, poverty, exile, death. None of these things is intrinsically glorious; but nothing can be glorious apart from them.
Note Seneca's observation in the last sentence. We are not to ignore and shun indifferents. Rather, they are the medium though which we demonstrate excellence. Indifferents hold no intrinsic value. What is of value is our use of indifferents in order to demonstrate our excellence of character (or not).
He provides examples:
For it is not poverty that we praise, it is the man whom poverty cannot humble or bend. Nor is it exile that we praise, it is the man who withdraws into exile in the spirit in which he would have sent another into exile. It is not pain that we praise, it is the man whom pain has not coerced. One praises not death, but the man whose soul death takes away before it can confound it.
Both Cato and Brutus died. But one exercised excellence and the other disgrace.
the death which in Cato's case is glorious, is in the case of Brutus forthwith base and disgraceful.
Seneca offers more examples of indifferents and correct (or incorrect) use of them.
thus it is with the things which we call indifferent and "middle," like riches, strength, beauty, titles, kingship, and their opposites, – death, exile, ill-health, pain, and all such evils, the fear of which upsets us to a greater or less extent; it is the wickedness or the virtue that bestows the name of good or evil.
Preparing for one's death cannot be actually practiced. Therefore, we must use all other manner of thought experiments to prepare for death; this is the practice of memento mori - or constantly recalling that we are mortal and that we must always keep this fact in the forefront of our minds - in order to meet death with neither fear nor trepidation, so that we can rationally choose the manner and attitude in which we will die.
The soul must be hardened by long practice, so that it may learn to endure the sight and the approach of death.
We must do everything willingly, or else it will count for naught. True virtue is choosing the wise course of action for the correct reasons. When it comes to true philosophy, the sage is one who willingly complies with Fate and Fortune for the right reasons. This is why noble lies are of no use. We must face reality and Nature without a veil. Preachers, teachers and leaders who intend to deceive in order to trick people into the right course of action, do many a disservice.
nothing glorious can result from unwillingness and cowardice; virtue does nothing under compulsion. Besides, no deed that a man does is honourable unless he has devoted himself thereto and attended to it with all his heart. ... virtue accomplishes its plans only when the spirit is in harmony with itself.
Because so many fear death and wish to cline to life, convincing people to prepare for death is difficult. This is hard mental work. But if one puts in the time and effort to prepare for death and to shun it, our excellence will be equal to the heroes of Sparta - the 300.
I point out to you the Lacedaemonians in position at the very pass of Thermopylae! They have no hope of victory, no hope of returning. The place where they stand is to be their tomb. ... Leonidas: how bravely did he address his men! He said: "Fellow-soldiers, let us to our breakfast, knowing that we shall sup in Hades!" ... It is not the Three Hundred, – it is all mankind that should be relieved of the fear of death.