The topic of this letter is "whether the Good is grasped by the senses or by the understanding; and the corollary thereto is that it does not exist in dumb animals or little children."
In sum, the Good can only be grasped and understood by reason. Animals and little children (and to some extent, non-adults), cannot attain the Good. This is related to my previous post on letter 123, where I stated "to completely grow and progress into the full bloom of a rational, human adult (logikē psuchē, see Sellars 104-105)." Animals do not even possess the capacity to reach the cohesion state of logikē psuchē. But children at least have the capacity to reach that cohesion state.
A rational human being, then, will contain pneuma at all four levels of tension. She will have pneuma as hexis giving cohesion to her bones, for instance; pneuma as phusis by virtue of being alive in the most basic biological sense; pneuma as psuchē giving her the animal faculties of impression and impulse; and pneuma as logikē psuchē giving her the rational power of judgement that can intervene between receiving impressions and acting on impulses.
Seneca notes that Stoics associate the Good with the rational mind and not the senses or pleasure.
Those who rate pleasure as the supreme ideal hold that the Good is a matter of the senses; but we Stoics maintain that it is a matter of the understanding, and we assign it to the mind.
we condemn men who are slaves to their appetites and their lusts, and we scorn men who, through fear of pain, will dare no manly deed.
The Stoics maintain that eudaimonia is obtained only through reason.
Reason, however, is surely the governing element in such a matter as this; as reason has made the decision concerning the happy life, and concerning virtue and honour also, so she has made the decision with regard to good and evil.
And eudaimonia is attained only according to Nature and that humans are born the the capacity to reach it.
we define as "happy" those things that are in accord with Nature. And that which is in accord with Nature is obvious and can be seen at once – just as easily as that which is complete. That which is according to Nature, that which is given us as a gift immediately at our birth, is, I maintain, not a Good, but the beginning of a Good.
Reason can only be attained and understood by rational humans. It cannot be attained by animals or children.
In that which does not possess reason, the Good will never exist. In that which is not yet endowed with reason, the Good cannot be existent at the time.
the Good cannot be discovered in any random person, or at any random age
Like wheat, the purpose of adult humans cannot be achieved until it is in full bloom.
There is a certain Good of wheat: it is not yet existent, however, in the swelling stalk, nor when the soft ear is pushing itself out of the husk, but only when summer days and its appointed maturity have ripened the wheat. Just as Nature in general does not produce her Good until she is brought to perfection, even so man's Good does not exist in man until both reason and man are perfected.
The Good is:
a free mind, an upright mind, subjecting other things to itself and itself to nothing
a matter of the understanding
a clear and flawless mind, which rivals that of God, raised far above mortal concerns, and counting nothing of its own to be outside itself. You are a reasoning animal. What Good, then, lies within you? Perfect reason.
How can you know when you have attained eudaimonia?
Only consider yourself happy when all your joys are born of reason, and when – having marked all the objects which men clutch at, or pray for, or watch over – you find nothing which you will desire; mind, I do not say prefer.
And Seneca offers this rule of thumb for know when you have reached that point.
"You will come to your own when you shall understand that those whom the world calls fortunate are really the most unfortunate of all."
Sellars, John. Stoicism. Berkeley, University Of California Press, 2006.