"A New Model of the Universe: Principles of the Psychological Method in Its Application to Problems of Science, Religion and Art". Book by P.D. Ouspensky, 1931.
Letter to Professor Christoffer Hansteen (1826) "Oeuvres Complètes de N. H. Abel", mathematician, Nouvelle edition (1881) ed., Peter Ludwig Mejdell Sylow & Sophus Lie, Vol. 2, pp. 263-265, as quoted by Øystein Ore, "Niels Henrik Abel: Mathematician Extraordinary", p. 113, 1957.