From "Essay on Criticism" (1711 AD) by Alexander Pope (1688-1744).
There is no evidence that Ellen G. White ever quoted or paraphrased this sentiment, though there is some evidence which seems to indicate she would have approved of it: "There is a medium position in these things. Oh, that we all might wisely find that position and keep it." (Testimonies for the Church, Vol. 1, p. 425:3)