Seneca · Letter IX
The Supreme Good needs no practical help from outside. It is grown at home and arises wholly within itself. The moment it seeks any part of itself from beyond, it falls under Fortune's sway.
What this means
The highest good, for Seneca, is self-sufficient, grown at home and complete within itself. The instant it needs something from outside, it surrenders to Fortune, who can give and take at will. A good that depends on luck is a good you can lose; the Stoics wanted one you cannot.
On fate, control, reason.