1923 – 2015
Georg Kreisel (1923–2015) was an Austrian-British mathematical logician whose work bridged formal proof theory and the philosophy of mathematics. He made foundational contributions to constructive analysis, ordinal logics, and the philosophical interpretation of Gödel's results. Kreisel was renowned for his insistence on 'informal rigor'—the precise articulation of pre-formal mathematical concepts before formalization.
Developed the 'squeezing argument' showing that informal and formal notions of logical validity coincide
Pioneered the use of proof-theoretic methods to extract computational content from classical proofs
Contributed to the philosophical analysis of Gödel's incompleteness theorems and their limits
Advanced the study of ordinal logics and transfinite progressions of formal systems
Argued for 'informal rigor' as the correct methodology for foundations of mathematics