[Aug 1978]
International Congress of Mathematicians, Helsinki, Finland.
[Aug 1979]
International Congress for Logic, Methodology and Philosophy
of Science, Hanover, Germany.
[Aug 1991]
International Congress for Logic, Methodology and Philosophy
of Science, Uppsala, Sweden.
[Aug 1995]
International Congress for Logic, Methodology
and Philosophy of Science, Florence, Italy.
[June, 2000]
Plenary Speaker at Association for Symbolic Logic
Centennial Meeting, Urbana, IL.
[June, 2007]
Plenary Speaker at Computability in Europe(CiE),
Computation and Logic in the Real World, Siena, Italy.