  Paul Lorenzen, mathematician and philosopher of the 20th century, mentions
October 1947 as the date of a crisis in his mathematical and philosophical
investigations. An autograph dated 15 October 1947 documents this crisis. This
article proposes a traduction and a commentary of it and sketches the
circumstances of its writing on the base of his correspondence with Paul
Bernays. A lettter from Lorenzen to Carl Friedrich Gethmann dated 14 January
1988 carves out the story of this crisis by showing how he soaks up the
indications of his correspondents and transmutes them into an absolutely
original research.
