Several years ago, I learned what the name of the shoe company really meant: asics
a - anima
s - sana
i - in
c - copore
s - sano
This is Latin for, sound mind, in a sound body. That has stayed with me for a long time. And I think that is ultimately what Seneca is trying to convey in Letter 15.
There is a happy medium between advancing the mind and maintaining a healthy body. One can tip the scales in one direction and spend an inordinate amount of time in the weight room. Vice versa, one can spend too much time with his nose in a book. The golden mean would suggest treating the body with its due diligence, while persisting in growing in wisdom, neither at the expense of the other.
Seneca expresses this idea, when he wrote:
Without philosophy the mind is sickly, and the body, too, though it may be very powerful, is strong only as that of a madman or a lunatic is strong.
He goes on to explain the mindlessness of heavy eating and drinking, followed by heavy exercise. The beasts do as much. But we are not beasts. We are rational beings and we ought to give the body its due diligence, so as to give ourselves the best time in the pursuit of knowledge and wisdom.
In our post-modern society, many have sacrificed both mind and body. Going to the mall to shop and spend time with friends was a memorable pass-time in my youth. But over the years, I can think of a handful of tortures I'd rather endure than spend time at the mall. Sometimes, in the last few years, the demands of family time and Christmas shopping caused me to sit in contemplation on a bench in the middle of the mall, while my wife was shopping. During these times, I took a poll of how many obese people walked past me and how many people were walking, head-down, staring into their smart phone. It was disheartening. Very, very few were not obese and very few were not staring into a phone. I wondered if our post-modern society has failed when it comes to educating people on obtaining sound health and sound minds.
What is the right balance? I guess it depends on the person. But the Mayo clinic recommends about 30 minutes of aerobic exercise per day, coupled with some resistance training (link). Once your exercise is complete, get back to the reading, writing and learning.
But whatever you do, come back soon from body to mind. The mind must be exercised both day and night, for it is nourished by moderate labour; and this form of exercise need not be hampered by cold or hot weather, or even by old age. Cultivate that good which improves with the years.
Do you have to be either in a state of exercise or a state of study at all times? No.
Of course I do not command you to be always bending over your books and your writing materials; the mind must have a change, – but a change of such a kind that it is not unnerved, but merely unbent.
For this reason, I would recommend when you are not exercising, learning or working, you should take up an activity that "unbends" the mind and refreshes it. This would be a hobby.
In closing, Seneca shares a couple of quotes about living in the present and being content.
"The fool's life is empty of gratitude and full of fears; its course lies wholly toward the future."
for we are plunged by our blind desires into ventures which will harm us, but certainly will never satisfy us; for if we could be satisfied with anything, we should have been satisfied long ago; nor do we reflect how pleasant it is to demand nothing, how noble it is to be contented and not to be dependent upon Fortune.
If you would thank the gods, and be grateful for your past life, you should contemplate how many men you have outstripped.
As to what the future's uncertain lot has in store, why should I demand of Fortune that she give, rather than demand of myself that I should not crave?