Princess Elisabeth of Bohemia began her correspondence with Descartes very tentatively on May 6, 1643, but Descartes, always enthusiastic to find some notable person interested in his philosophical work, quickly set her mind at ease; they would go on to have a fruitful correspondence over the next several years, discussing the relation of mind and body (the first topic Elisabeth brought up), mathematics, medicine, politics, and the difficulties of doing philosophy on a busy schedule. Among all these topics, one of the interesting discussions was a brief look at Seneca's De vita beata. (It is important to note that the chronological order of the letters can be somewhat misleading, despite its convenience, because there are several instances where it seems clear that their letters are passing each other en route, so that they are not responding to the other person's most recent letter, which they have not yet received.)
The discussion begins well into their correspondence, with the letter from Descartes to Elisabeth for July 21, 1645. Descartes notes that he wants nothing more than to see the Princess happy, and so concludes that the subject that seems worth discussing is "how philosophy teaches us to acquire this sovereign felicity which vulgar minds vainly expect from fortune, but which we can obtain only from ourselves" (252 / 96). He suggests that looking at what the ancients have said on the subject is the best way of discussing the question, so he recommends starting with Seneca's De vita beata, if she does not prefer doing another text. He explicitly asks for her own comments on the book, for his how instruction.
In the very next letter, however, that of Descartes to Elisabeth for August 4, he already regrets his choice. He had chosen it, he says immediately, due to the reputation of the book, but having read it, he has concluded that Seneca's approach is not exact enough to make it worth doing. He sees this, however, as an opportunity for explaining how he thinks Seneca should have approached the subject.
He starts with the question of what is meant by vivere beate, and faces a problem with translating it into French (the same problem we have translating it into English): it is natural to translate it as "to live happily (heureusement)" but this runs the danger of suggesting that it is about fortune (l'heur) rather than about what is really meant (la béatitude). To clear this up he gives his own preferred account of that in which la béatitude consists: "a perfect contentment of the mind and an internal satisfaction that those who are the most favored by fortune ordinarily do not have and that the sages acquire without fortune's favor" (264 / 97).
The next question that needs to be considered is what causes this contentment and satisfaction. Descartes identifies two things: first, some causes depend on us (like virtue or wisdom) and some don't (like honors and riches). But the fact that the former category even exists means that our happiness does not have to depend on fortune. Thus, he concludes, three things seem to allow us a reasonable guarantee of happiness:
(1) Trying to make use of one's mind as well as one can.
(2) Having a firm resolution to do what reason advises; this firmness of resolution is virtue.
(3) Accustoming oneself not to desire the causes of happiness that are outside of one's power, but to focus on those things that are within one's power.
Descartes notes explicitly that these three rules are related to the "provisional code of morals consisting of three or four maxims" that he had given in Part III of the Discourse on Method. This suggests that the provisional code might perhaps be more important than the immediate context of the Discourse makes clear. Descartes's identification of virtue with firmness of resolution in the pursuit of rational life is also interesting, and it is clear from how he describes it that he recognizes that this account of virtue is somewhat unusual.
Only desires involving impatience and sadness are inconsistent with b´atitude; likewise, reason doesn't have to be always right for us to have it. So the only thing we need in order to have genuine happiness is virtue, which is firmness in acting according to right reason. But this does require that our intellect take some trouble to make clear what virtue is.
This, then, is what Seneca should have done: laid out the basic truths that make it possible to recognize virtue and follow it.
Elisabeth more or less agrees in her letter to Descartes of August 16. As she notes, she found the book more useful for providing topics for reflection than for understanding what the happy life is. The problem is that Seneca has no method, and so instead of actually describing béatitude, he just defends the possibility of achieving it even if you are wealthy. She encourages Descartes to continue his analysis of Seneca, not because she finds it surprising, but because he is expressing naturally what seems to her to be right, thus allowing her to have a better understanding of how everything fits together.
She doubts, however, that it is possible to have happiness without any reliance at all on things out of our power. Some diseases interfere with reason, for instance, and so eliminate that satisfaction; others make it difficult to follow the maxims we need to follow in order to achieve happiness. When Epicurus tried not to show pain when he was dying because of kidney stones, he was able to do so because he was a philosopher; a prince or courtier would not be in a situation that would make it possible.
Descartes, never averse to giving his own philosophical views, accedes to Elisabeth's request in his 18 August letter to her. Having discussed what Seneca should have done, he will not critique what Seneca actually did. There are three basic things that he attempts:
(1) He tries to give definitions of sovereign good.
(2) He tries to argue against the Epicureans.
(3) He responds to those who claim philosophers do not practice what they preach.
With regard to the first, Seneca insists on the importance of reason over custom; Descartes agrees entirely, although he thinks Seneca's formulations are often not very exact. He is unimpressed with the definitions Seneca gives, however. In particular, he thinks it is unclear what Seneca means by 'nature' when he says that we should live in accord with nature and with our own nature. He seems to mean the order established by God. But, Descartes says, this seems to leave everything unexplained. Further, since Seneca gives several definitions, it suggests that he himself might not have a clear idea of what he is trying to say.
With regard to the second, Descartes begins by setting forth his own view. First, he insists that sovereign good, béatitude, and the goal to which actions ought to tend are all distinct. Béatitude presupposes sovereign good, but is the actual possession of it; and the goal to which our actions should tend could be either, since sovereign good is what we should seek and béatitude is what attracts us to sovereign good.
He then suggests that there is an equivocation in criticisms of Epicurus. Critics of Epicurus claim that by 'pleasure' he means only sensible pleasure; in reality it is clear from what Seneca and others say that he actually held that it was any kind of contentment of mind. When we get this cleared away, we have three ancient views of sovereign good: Aristotle takes it to be all perfections of body and mind; Zeno takes it to be virtue; and Epicurus takes it to be pleasure. All of these are true if understood a certain way. Aristotle is essentially right, but Zeno and Epicurus are more immediately relevant here. Zeno is entirely right if we think of what a person can have on his or her own power. But this also makes it look so severe that only people of a particular temperament could go along with it. Epicurus, on the other hand, is also right, since even virtue wouldn't make us happy if we had no pleasure in virtue. But the problem is that this way of talking about sovereign good seems to obscure the importance of virtue.
Thus Descartes prefers his own account of béatitude as contentment in general; even if there are bodily causes of contentment, the only solid path would be virtue, since it is in our power.
Elisabeth in a further letter of August is pleased with Descartes's exposition and reiterates again that the obscurity of ancient authors comes from their lack of method. Seneca seems to treat of the Epicurean philosophy "more as a satirist than as a philosopher" (280 / 106). She is especially pleased at his account of how all the major ancient positions can be right, since it serves as an answer to a possible skeptical objection -- namely, that because they disagree the sovereign good must be difficult to find. She encourages him to continue.
In his letter of September 1, Descartes continues his discussion by addressing Elisabeth's August 16th worry about how much happiness actually is in our power. He agrees that there are diseases that take away the power of reasoning and therefore the possibility of rational satisfaction, and he concludes that what he had applied to everyone should actually apply only to those who have free use of reason and know how to reach happiness. Everyone wants to be happy, but people don't always know how, and our bodies may interfere with our ability to think through what we are doing. He compares this to sleep: you can be as rational as you please, and still have irrational bad dreams when sleeping. Nonetheless, if one has any free use of one's mind, one can train one's mind to return to things that bring contentment. And other indispositions, while they may make things more difficult, are nonetheless things we can overcome.
We also, however, need to know something about the causes of contentment, which is the same kind of knowledge required for virtue: "For all the actions of the mind which bring us some perfection are virtuous, and all our contentment consists only in our inner testimony of having some perfection" (283-284 / 107). All exercise of virtue, therefore, brings some pleasure. But these pleasures are not all the same. Pleasures of the mind insofar as it is united to the body, for instance, are confused and so we can misjudge how great the pleasure will be, or how great the perfection associated with it might be. This shows even more clearly the need for reason to evaluate all our pleasures. What we will find is that the pleasures of the body are often lesser, and associated with lesser perfections, and not as lasting; and thus in this sense we can say that what really matters are pleasures of the mind itself, which can be as stable as reason. But, he hastens to add, we should not despise the pleasures of the body; the point is to subject them to reason, not eliminate them entirely.
Elisabeth's next letter, of September 13, will turn discussion more closely to the role of the passions in all of this, and Descartes in his letter of September 15 will agree with Elisabeth's diagnosis of Seneca in the letter in which she says he writes more like a satirist than a philosopher, and they will therefore go on to discuss Descartes's own ideas much more closely. Thus the discussion of the sovereign good will continue; but, as this is more or less all they say about Seneca's De vita beata, this is where we will leave off.
Quotations are from Princess Elisabeth of Bohemia and Rene Descartes, The Correspondence between Princess Elisabeth of Bohemia and Rene Descartes, Lisa Shapiro, ed. & tr. The University of Chicago Press (Chicago: 2007).
References are: (Adam-Tannery page number for Descartes's works / page number in Shapiro's translation).