The day you stop learning is the day you begin decaying, and then you are no longer a human being.

Isaac Asimov (1920-1992) Russian-American author, polymath, biochemist

Commencement speech, Connecticut College (1975-05-25)
    (Source)

Quoted in Peter Smith, ed., Onward! 25 Years of Advice, Exhortation, and Inspiration from America's Best Commencement Speeches (2000).