The first letter I read was on Self Control. This letter was about your emotions and how they can become vices if you become overindulged in them. No matter what, people will experience certain feelings and desires that they can't always control. But, if you act on these emotions and let yourself become overindulged in them sometimes it is hard and almost impossible to make them go away. What Seneca says is ideal, but also hard to do, is being able to let yourself indulge in emotions and give into certain desires only in moderation. This letter relates to the philosophical technique of Dichotomy of Control. Our thoughts which can dictate our emotions and desires are not always up to us and are out of our control. But what is up to us is how we choose to act on those thoughts and desires.
The second letter I read was On Grief for Lost Friends. This letter was about the line between what is a normal amount of grieving for the death of a friend and what is considered an excessive and unnecessary amount. Seneca says he "would not have you sorrow more than is fitting". Seneca points out that humans are mortal beings and at any moment their time on Earth can run out. This letter relates to the philosophical technique of Negative Visualization because if you visualize what your life would be like if you lost a good friend, it will help your realize and appreciate them more while they are still alive. If and when they do pass away, because everyone eventually does, you will not dwell and grieve for an unnecessarily long period of time, because you already accepted that it was going to happen. And, because of negative visualization you didn't take them for granted while they were still alive.