When Thales was asked what was difficult, he said, "To know one's self." And what was easy, "To advise another."

Diogenes Laertius (Circa 200 a d): Thales. ix.

  O eloquent, just, and mightie Death! whom none could advise, thou hast perswaded; what none hath dared, thou hast done; and whom all the world hath flattered, thou only hast cast out of the world and despised. Thou hast drawne together all the farre stretchèd greatnesse, all the pride, crueltie, and ambition of man, and covered it all over with these two narrow words, Hic jacet!

Sir Walter Raleigh (1552-1618): Historie of the World. Book v. Part 1.