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).