He who is content with what has been done is an obstacle in the path of progress.

Helen Keller (1880–1968) American author and lecturer

“Our Duties to the Blind,” speech, Massachusetts Assoc. for Promoting the Interests of the Adult Blind, Boston (1904-01-05)
    (Source)

Reprinted in Out of the Dark: Essays, Lectures, and Addresses on Physical and Social Vision (1907).