According to the proverb, the best things are the most difficult.

Plutarch (46(?)-120(?) a d): Of the Training of Children.

  Many things difficult to design prove easy to performance.

Samuel Johnson (1709-1784): Rasselas. Chap. xvi.

  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.