“Tell a Man What He May Not Sing”

In her novel, The Praise Singer (1978), Mary Renault has the poet Simonides, her narrator, talk of free speech:

Tell a man what he may not sing, and he is still half free; even all free, if he never wanted to sing it.  But tell him what he must sing, take up his time with it so that his true voice cannot sound even in secret—there, I have seen, is slavery.