It seems Lucilius is looking for proofs in the forms of syllogisms, to support the Stoic claim that living a virtuous life leads to a happy life. While reluctant to do so, Seneca does offer a few arguments to support the main claim.
I was satisfied to give you a sort of taste of the views held by the men of our school, who desire to prove that virtue is of itself sufficiently capable of rounding out the happy life. But now you bid me include the entire bulk either of our own syllogisms or of those which have been devised by other schools for the purpose of belittling us. If I shall be willing to do this, the result will be a book, instead of a letter. And I declare again and again that I take no pleasure in such proofs.
The first one he tackles is this:
"He that possesses prudence is also self-restrained; he that possesses self-restraint is also unwavering; he that is unwavering is unperturbed; he that is unperturbed is free from sadness; he that is free from sadness is happy. Therefore, the prudent man is happy, and prudence is sufficient to constitute the happy life."
The Peripatetics do not argue for the abolishment of passions, but rather for the moderation of them. Seneca disagrees and argues against the notion of simply trying to be better than the least common denominator. He argues for in favor of striving towards sage hood.
how petty is the superiority which we attribute to the wise man, if he is merely braver than the most craven, happier than the most dejected, more self-controlled than the most unbridled, and greater than the lowliest! ... This is speed estimated by its own standard, not the kind which wins praise by comparison with that which is slowest. Would you call a man well who has a light case of fever? No, for good health does not mean moderate illness.
He later states,
I am not referring to the gradual weeding out of evils in a good man, but to the complete absence of evils; there should be in him no evils at all, not even any small ones.
As I read Seneca's argument on this, I am reminded of the sentiment of aiming for the stars when you wish to go to the moon.
The aim for a practicing Stoic and one who is making progress, is to be a sage. And while very few may be considered a sage, it nonetheless is a worthy goal.
He then makes a number of arguments for putting down passions and vices urgently and with focus, rather than making little progress across multiple passions and vices.
a throng of such, even though they be moderate, can affect him more than the violence of one powerful passion. ... We could deal better with a person who possessed one full-fledged vice, than with one who possessed all the vices, but none of them in extreme form.
And once you break the habit and over-come passions and vice, you must never let them in again.
Tigers and lions never put off their wildness; they sometimes moderate it, and then, when you are least prepared, their softened fierceness is roused to madness. Vices are never genuinely tamed. Again, if reason prevails, the passions will not even get a start ... it is easier to stop them in the beginning than to control them when they gather force. This half-way ground is accordingly misleading and useless; it is to be regarded just as the declaration that we ought to be "moderately" insane, or "moderately" ill.
Therefore, work to remove them wholly, and then keep them out.
You can more easily remove than control them. ... it is easier to keep a thing out than to keep it under after you have let it in.
This idea leads to the next syllogism:
"If a man has self-control and wisdom, he is indeed at peace as regards the attitude and habit of his mind, but not as regards the outcome. For, as far as his habit of mind is concerned, he is not perturbed, or saddened, or afraid; but there are many extraneous causes which strike him and bring perturbation upon him."
This is a recognition of external or "extraneous" causes which may bring fear to the doorstep of his inner citadel. If the man never allows them (fear, passions) into his mind, he retains self-control and wisdom.
Taking this approach to happiness, a man may retain self-control, temperance and courage, regardless of what lands on his doorstep. And if this is true, then he may be content and happy with life and what is up to him.
since the happy life contains in itself a good that is perfect and cannot be excelled, if a man has this good, life is completely happy. Now if the life of the gods contains nothing greater or better, and the happy life is divine, then there is no further height to which a man can be raised. Also, if the happy life is in want of nothing, then every happy life is perfect ... the Supreme Good does not admit of increase ... so the happy life cannot be increased either.
He then clarifies a notion, in which many do err. If you always want something else, how can you be happy?
the more prudent he is, the more he will strive after the best, and he will desire to attain it by every possible means. But how can one be happy who is still able, or rather who is still bound, to crave something else?
He rectifies the erroneous thinking:
men do not understand that the happy life is a unit; for it is its essence, and not its extent, that establishes such a life on the noblest plane. Hence there is complete equality between the life that is long and the life that is short, between that which is spread out and that which is confined, between that whose influence is felt in many places and in many directions, and that which is restricted to one interest. Those who reckon life by number, or by measure, or by parts, rob it of its distinctive quality.
This is a profound concept and one which Pierre Hadot discusses in his book Philosophy as a Way of Life. Hadot quotes Nietzsche (p. 235):
Let us assume we say "Yes!" to one single, unique moment: we have thus said yes, not only to ourselves, but to the whole of existence. For nothing is isolated, neither in ourselves nor in things. And if, even once, our soul has vibrated and resounded like a string with happiness, all eternity was necessary to created the conditions for this one event; and all eternity has been approved, redeemed, justified, and affirmed.
Seneca gets more specific about happiness:
Now, in the happy life, what is the distinctive quality? It is its fulness. ... Satiety.
Think about this - do you want to strive towards a limitless list of desires, pursuing a life by checking off desires one by one? Where does it end? Will you ever be happy? Or could you be highly discriminating in your pursuit of the most important desire and pursue it? If you had one desire which brought happiness or an infinite list of desires which may bring happiness, which would you choose?
The answer, as I understand it, is to place your desires on pursuing an excellent character, so as to be beyond the grasp of Fortune and indifferents. Regardless of having health or illness, wealth or poverty, life or death, you could attain satisfaction and happiness based on your attitude and virtuous soul - this is what Stoicism proposes.
The next syllogism Seneca discusses is this:
"He who is brave is fearless; he who is fearless is free from sadness; he who is free from sadness is happy."
But fearful of what? Other schools focus on fear of evils and herein we need to unpack what is evil and what is not.
Others may say evils are pain, torture and misfortune. But to a Stoic sage, evils are not those things. The sage "believes that the only evil is baseness" or assuming an unvirtuous character.
death, imprisonment, burning, and all the other missiles of Fortune... are not evils.
To the Stoic sage, if you
Paint him a picture of slavery, lashes, chains, want, mutilation by disease or by torture, – or anything else you may care to mention; he will count all such things as terrors caused by the derangement of the mind. These things are only to be feared by those who are fearful.
He further clarifies what are evils to the Stoic sage.
It is the yielding to those things which are called evils; it is the surrendering of one's liberty into their control, when really we ought to suffer all things in order to preserve this liberty. Liberty is lost unless we despise those things which put the yoke upon our necks. If men knew what bravery was, they would have no doubts as to what a brave man's conduct should be. For bravery is not thoughtless rashness, or love of danger, or the courting of fear-inspiring objects; it is the knowledge which enables us to distinguish between that which is evil and that which is not.
Allowing indifferents to control your state of mind - your happiness - is evil. It is letting the idea of achieving riches, wealth, fame, glory or letting the idea of trying to avoid poverty, destitution, ignominy, dishonor, control your state of mind.
And to be precise and whole, Seneca also notes this on emotions and feelings.
Yes, he has felt pain; for no human virtue can rid itself of feelings. But he has no fear; unconquered he looks down from a lofty height upon his sufferings.
He continues with the next syllogism, which is related to the preceding point.
"That which is evil does harm; that which does harm makes a man worse. But pain and poverty do not make a man worse; therefore they are not evils."
After using an analogy of a good helmsman or pilot, Seneca concludes the thought with this:
The wise man's purpose in conducting his life is not to accomplish at all hazards what he tries, but to do all things rightly. ... And the more he is hampered by the stress of fortune, so much the more does his knowledge become apparent.
And then he writes this excellent passage on right use of a universe full of indifferents:
the wise man is not harmed by poverty, or by pain, or by any other of life's storms. For all his functions are not checked, but only those which pertain to others; he himself is always in action, and is greatest in performance at the very time when fortune has blocked his way. For then he is actually engaged in the business of wisdom; and this wisdom I have declared already to be, both the good of others, and also his own. Besides, he is not prevented from helping others, even at the time when constraining circumstances press him down. Because of his poverty he is prevented from showing how the State should be handled; but he teaches, none the less, how poverty should be handled. His work goes on throughout his whole life. Thus no fortune, no external circumstance, can shut off the wise man from action. For the very thing which engages his attention prevents him from attending to other things. He is ready for either outcome: if it brings goods, he controls them; if evils, he conquers them. So thoroughly, I mean, has he schooled himself that he makes manifest his virtue in prosperity as well as in adversity, and keeps his eyes on virtue itself, not on the objects with which virtue deals.
We moderns love Marcus Aurelius' quote regarding the 'obstacle is the way', which has been popularized by Ryan Holiday. But the word "obstacle" causes some to trip up, perhaps. It implies the traveler is on a path and there is an obstruction in his path and all that the traveler wants to do is continue on the path. To slightly alter the analogy, we need to get into the attitude and head of the traveler. Instead of thinking "all the traveler wants to do is continue on the path", we change her attitude to "I wish to show the world how well I travel on this path, whatever it throws in my path." Now, instead of an obstacle in the way, that big rock is an opportunity for her to show and demonstrate her ability to rock climb. Now, no matter what is on the path, or if there is sunshine or rain, she demonstrates skill and excellence of attitude, in all circumstances.
In my opinion, the clue to realizing you do not have the correct mindset is to recognize when you are complaining. The sage does not complain about exile, poverty, illness or death. Nor is he overjoyed (or complaining) when he has health, wealth, fame and is alive. The word - the goal - to strive for is equanimity.
So the wise man will develop virtue, if he may, in the midst of wealth, or, if not, in poverty; if possible, in his own country – if not, in exile; if possible, as a commander – if not, as a common soldier; if possible, in sound health – if not, enfeebled. Whatever fortune he finds, he will accomplish therefrom something noteworthy. ... the wise man is a skilled hand at taming evils. Pain, want, disgrace, imprisonment, exile, – these are universally to be feared; but when they encounter the wise man, they are tamed.