Seneca begins the letter with a bold statement.
no man can live a happy life, or even a supportable life, without the study of wisdom; you know also that a happy life is reached when our wisdom is brought to completion, but that life is at least endurable even when our wisdom is only begun.
Said a bit differently, people who don't study philosophy aren't truly happy. For my part, I fall in the latter camp, where I've only begun to study philosophy, so I can at least endure life.
Living a life of wisdom is like brushing your teeth: it needs to be performed daily and not quickly. You cannot expect the dentist to give you a pass if you never brush your teeth every day, but then on the morning of the day you go to the dentist, you brush for five minutes expecting to get all the decay off. No, you must learn and practice it every day.
This idea, however, clear though it is, must be strengthened and implanted more deeply by daily reflection; it is more important for you to keep the resolutions you have already made than to go on and make noble ones.
Seneca also notes that habits kept are habits made. You must not go on to setting goals of new habits until you've established the ones you've already committed to.
Through daily reflection, you must introspect if you are passing the test or merely passing time.
Examine yourself; scrutinize and observe yourself in divers ways; but mark, before all else, whether it is in philosophy or merely in life itself that you have made progress.
An alternate translation of the above passage notes the subtle difference:
Carry out a searching analysis and close scrutiny of yourself in all sorts of different lights. Consider above all else whether you’ve advanced in philosophy or just in actual years.
I don't often come across the God or atoms debate when I read Seneca. Marcus alludes to it quite often. But in this passage, Seneca puts a slightly different perspective on the debate.
Perhaps someone will say: "How can philosophy help me, if Fate exists? Of what avail is philosophy, if God rules the universe? Of what avail is it, if Chance governs everything? For not only is it impossible to change things that are determined, but it is also impossible to plan beforehand against what is undetermined; either God has forestalled my plans, and decided what I am to do, or else Fortune gives no free play to my plans."
In sum, Seneca asks if there is free will, weather God is calling the shots or if everything based on a flip of a coin. I didn't get any answers from Seneca, other than some consolation.
She [philosophy] will encourage us to obey God cheerfully, but Fortune defiantly; she will teach us to follow God and endure Chance.
To me, he saying "it's all in your attitude." If you think God's calling the shots, then follow your fate. If you think the coin determines your fate, then endure it well.
Seneca closes with a quote from Epicurus.
"If you live according to nature, you will never be poor; if you live according to opinion, you will never be rich." Nature's wants are slight; the demands of opinion are boundless.
Natural desires are limited; but those which spring from false opinion can have no stopping-point. The false has no limits. When you are travelling on a road, there must be an end; but when astray, your wanderings are limitless. Recall your steps, therefore, from idle things, and when you would know whether that which you seek is based upon a natural or upon a misleading desire, consider whether it can stop at any definite point. If you find, after having travelled far, that there is a more distant goal always in view, you may be sure that this condition is contrary to nature.
Very little is required to live a fulfilled life. But if your wants are insatiable (living according to opinion or what the neighbors just bought), then you'll never be satisfied.