'Aude sapere' is the subtitle to Samuel Hahnemann's Organon of Medicine. Hahnemann is believed to have first encountered the phrase – albeit in reverse ('SAPERE AUDE') – as a pupil at the Prince's Grammar School in his home town of Meissen in Saxony. Still visible today (see left), it is inscribed in stone above an entrance doorway in honour of the poet, Gotthold Ephraim Lessing, another former pupil of the school.

Already famous as a young man, Lessing had adopted the phrase as his own personal motto from the Roman poet, Horace. The latter had used it in his first book of Epistles: dimidium facti qui coepit habet: sapere aude, incipe ('He who has begun is half done: dare to know!'). It is the moral of a story in which a fool waits for a stream to stop before crossing it. The phrase may therefore be understood as: 'He who begins is half done. Dare to reach the other side.' 

Three years after Lessing's death, Immanuel Kant declared the phrase 'Sapere aude!' to be the motto of the Enlightenment in his 1784 essay, Answering the Question: What Is Enlightenment?