This was attributed to Seneca the philosopher. It may sound trite and vague.
What exactly did he mean?
Here is one possibility. Even in Seneca's days, people were aware of the cost of
borrowing money. The wise man might not be super-rich, but however much he had,
he lent it out and collected interests which he did not live beyond. He himself
did not have to work; his capital worked for him.
Today, US treasury bills yield over 5%.
The idea is simple; its execution anything but.