In 1955, Paul Lorenzen clears the sky in foundations of mathematics for Hermann Weyl

Abstract

In 1955, Paul Lorenzen is a mathematician who devotes all his research to foundations of mathematics, on a par with Hans Hermes, but his academic background is algebra in the tradition of Helmut Hasse and Wolfgang Krull. This shift from algebra to logic goes along with his discovery that his ``algebraic works [...] have been concerned with a problem that has formally the same structure as the problem of consistency of the classical calculus of logic'' (letter to Carl Friedrich Gethmann dated 14 January 1988). After having provided a proof of consistency for arithmetic in 1944 and published it in 1951, Lorenzen inquires still further into the foundations of mathematics and arrives at the conviction that analysis can also be given a predicative foundation. Wilhelm Ackermann as well as Paul Bernays have pointed out to him in 1947 that his views are very close to those proposed by Hermann Weyl in Das Kontinuum (1918): sets are not postulated to exist beforehand; they are being generated in an ongoing process of comprehension. This seems to be the reason for Lorenzen to get into contact with Weyl, who develops a genuine interest into Lorenzen's operative mathematics and welcomes with great enthusiasm his Einf{\"u}hrung in die operative Logik und Mathematik (1955), which he studies line by line. This book's aim is to grasp the objects of analysis by means of inductive definitions; the most famous achievement of this enterprise is a generalised inductive formulation of the Cantor-Bendixson theorem that makes it constructive. This mathematical kinship is brutally interrupted by Weyl's death in 1955; a planned visit by Lorenzen at the Institute for Advanced Study in Princeton takes place only in 1957--1958. As told by Kuno Lorenz, Lorenzen's first Ph.D. student, a discussion with Alfred Tarski during this visit provokes a turmoil in Lorenzen's operative research program that leads to his abandonment of language levels and to a great simplification of his presentation of analysis by distinguishing only between ``definite'' and ``indefinite'' quantifiers: the former govern domains for which a proof of consistency is available and secures the use of the law of excluded middle; the latter govern those for which there isn't, e.g. the real numbers. Lorenzen states in his foreword to Differential und Integral (1965) that he is faithful to Weyl's approach of Das Kontinuum in this simplification. This history motivates a number of mathematical and philosophical issues about predicative mathematics: how does Weyl's interest into Lorenzen's operative mathematics fit with his turn to Brouwer's intuitionism as expressed in ``{\"U}ber die neue Grundlagenkrise der Mathematik'' (1921)? Why does Lorenzen turn away from his language levels and how does this turn relate to Weyl's conception of predicative mathematics? What do Lorenzen's conceptions of mathematics reveal about Weyl's conceptions?