[1] Thoreau, Henry David. "The Mass of men lead lives of quiet desperation, and go to the grave with the song still in them"
[2] Dalam Carnegie, Dale. 1986. Petunjuk Hidup Tentram dan Bahagia. Jakarta : PT.Gramedia, hal. 121
[3] Arthur Scohpenhauer (1788-1860), Filosof Jerman yang terkenal dengan sikap pesimisnya tentang sifat manusia
[4] Hal. 120