A man of conscience and known ability should insist on full freedom to act as he in his own judgment deems best, and should not consent to serve on any other terms.
 John Stuart Mill, Considerations on Representative Government (1861). copy citation

Context

“but for the able man to lend himself to this compromise is treason against his especial office—abdication of the peculiar duties of mental supremacy, of which it is one of the most sacred not to desert the cause which has the clamor against it, nor to deprive of his services those of his opinions which need them the most. A man of conscience and known ability should insist on full freedom to act as he in his own judgment deems best, and should not consent to serve on any other terms. But the electors are entitled to know how he means to act; what opinions, on all things which concern his public duty, he intends should guide his conduct. If some of these are unacceptable to them, it is for him to satisfy them that he nevertheless deserves to be their representative;” source