On Living in the World -- from ONE EQUALL LIGHT (Anthology of John Donne writings collected by Dr. John Moses, Dean of St. Paul's Cathedral, London, England)

This excerpt is found in Volume 7 of Donne's Sermons (10-volume set published by Univ. of California Press, 1953 - 1962):
Upon this earth, a man cannot possibly make one step in a straight and a direct line..  The earth itself being round, every step we make upon it must necessarily have a segment, an arc of a circle.  But yet through no piece of a circle be a straight line, yet if we take any piece, nay, if we take the whole circle, there is no corner, no angle in any piece, in any entire circle.  A perfect rectitude we cannot have in any way in this world.  In every Calling there are some inevitable temptations.  But though we cannot make up our circle of a straight line (that is impossible to human frailty) yet we may pass on, without angles, and corners.  That is, without disguises in our Religion, and without love of craft and falsehood and circumvention in our civil actions. (Volume VII.9.272-83: adapted for current spelling and punctuation)