- José Ortega y Gasset; Julián Marías. Meditations on Quixote. trans. Evelyn Rugg and Diego Marín. University of Illinois Press. 2000: 85. ISBN 0-252-06895-5.
- According to Schopenhauer, this judgment was attributed to Aristotle. Schopenhauer presents the Latin version as Nihil est in intellectu nisi quod antea fuerit in sensu. See The World as Will and Representation, Volume II, Chapter VII. It is possible that it was mentioned by the Stoic Cicero and was repeated by Augustine of Hippo and Thomas Aquinas.