The wise person is still not harmed by the storms of life—poverty, pain, and the rest. For not all his works are hindered but only those that pertain to others. He is himself, always, in his actions, and in the doing of them he is greatest when opposed by fortune. For it is then that he does the business of wisdom itself, which as we just said is his own good as well as that of others. (Letters 85.37)
Fortuna, for Seneca, is not an anthropomorphized divinity with malicious intentions. Instead, Fortuna (fortune) is a metaphor for those events in life which appear to hinder or help us achieve our desires and intentions. Fortuna is the slow driver in front of us, making us late for work or school, the overbearing boss, the unexpected bill, the life-threatening medical diagnosis, the termination letter from an employer, the breakup of a relationship, etc.

Alternatively, as Seneca points out, Fortuna may masquerade as an apparent good, tempting us to succumb to desires and aversions outside of our control. The appearance of good fortune may include lottery winnings, promotion, fifteen minutes of fame, a new lover, etc. As we can see, Fortuna can present herself as either an apparent good or an apparent evil when, in fact, she is neither.

Fortuna is a metaphor for the externals outside our control and serves as grist for our character's mill. As such, those external circumstances, which Seneca labels Fortuna, are indifferents that have no inherent ability to affect our moral character (virtue). Nevertheless, they are the very things and events that challenge us and allow us to develop our moral character toward excellence. Without the challenges offered by Fortuna, we lack the means to develop our excellence of character fully. As Seneca points out:
In fair weather anyone can be a helmsman. (Letters 85.34)
Our character is not challenged and developed when the seas of life are smooth and the winds are calm and steady, blowing in the direction of our wishes. Instead, our character is tested and can thereby develop most rapidly, at those times when the sea becomes turbulent and blustering winds threaten to shred our sail. Therefore, the storms of life that threaten to drive the bow of our ship under the waves are the events that serve to test and strengthen our character. In Seneca’s words:
To fashion a man [or woman] who can genuinely be called a [Stoic], a stronger fate is needed. For him, the way will not be flat: he must go up and down, he must be tossed by waves, and must guide his vessel on a stormy sea. He must hold his course against fortune. Many things will happen that are hard and rough—but things he can soften and smooth out himself. Fire proves gold; misery, brave men [and women]. (On Providence 5.9)
When Fortuna stirs up a storm in your life and appears intent on driving your ship onto the rocks or into the depths, keep this truth from the Stoics in mind: Fortuna is not your enemy; she is your teacher. You can choose to welcome her into your life and learn the lessons she offers, or you can ignore the lessons of Fortuna, resist fate, and suffer the psychological consequences. We learn that Fortuna is not an existential threat by trusting the benevolence of a providential cosmos and focusing our attention on what is up to us.

Our struggle with Fortuna is not a fight against external circumstances. Instead, it is a struggle with our desire for circumstances to be other than they are. As we learned from Encheiridion 8, the goal of Stoicism training is learning to wish for things to happen as they do. Again, this does not mean we wish for dispreferred outcomes in advance; that’s not what the Stoics taught. However, when those events occur, we need to use them to develop our character.

Stoicism teaches us to look for the lesson in the storms of life. Fortuna may use a storm to redirect our ship toward a destination we did not originally intend. Alternatively, the squall we face today may prepare us for a m...