Logic List Mailing Archive

Theo Janssen (1948-2018)

Theo M.V. Janssen (29 July 1948 -- 6 December 2018)
http://www.illc.uva.nl/NewsandEvents/Obituaries/Theo-Janssen-1948-2018-/

Theo Janssen started his study in mathematics at the University of 
Amsterdam in 1966, obtaining a candidate's degree in 1969 and a master's 
degree in 1975. Starting from 1972, he was also a student assistant in the 
department of mathematics. From 1975 to 1980, he was a junior scientist in 
the pure mathematics division of the Mathematical Centre in Amsterdam on a 
project connecting computation and natural language, and from 1980 to 
1985, he held a position in the department of philosophy. Starting from 
1985, Theo became an assistant professor in the newly built department of 
computer science, joining the theory group of Peter van Emde Boas whose 
main topics were algorithmics and semantics. This group entered the ILLC 
officially in 1991, where Theo remained active until his retirement in 
2013.

Theo's master's thesis concerned two-level 'van Wijngaarden grammars'. 
This well-known formal grammar formalism was known to have universal 
computing power, but the reverse inclusion was still open. Theo proved 
that that no supercomputable power arises: two-level grammars compute no 
more than the standard computable functions.

However, from an early stage, Theo's interests broadened from the 
mathematics of formal grammar and computation to connections with the 
syntactic and semantic structures found in actual natural language. This 
led to his joining an Amsterdam circle of young linguists, logicians, and 
philosophers, emerging in the mid 1970s, which included many people who 
would later become founding members of the ILLC. This group focused its 
attention on the theory we know today as Montague Grammar which combines 
logical rigor with serious linguistic content. Theo's initial interest was 
a computer implementation of the grammar in Montague's paper "The proper 
treatment of quantification in ordinary English". This implementation was 
to be flexible and conceptually transparent, in order to cope with the 
various extensions of the original fragment proposed by many linguists at 
the time.

Soon this search for connections between mathematics and natural language 
acquired broader dimensions when Theo discovered further systematic 
parallels between natural language and computation. In 1976, he managed to 
capture the concept of a variable in programming languages using 
Montague's intensional logic. The resulting model-theoretic semantics, 
which solved existing problems in understanding crucial computational 
devices like arrays and pointers, was presented at various prestigious 
computer science conferences.

To many, this work of Theo Janssen's has shown that Montague's program 
does not just apply to natural and mathematical languages, but also to 
programming languages, thus considerably expanding its scope, by placing 
it in an interdisciplinary triangle of logic, language and computation. 
But he also went on to the foundations of this interface, finding powerful 
mathematical formulations of the principle of Compositionality, "the 
meaning of a compound expression is composed from the meanings of its 
parts", in terms of many-sorted algebras and homomorphisms.  These were 
then used as a yardstick for a (sometimes very) critical assessment of 
proposed linguistic extensions of Montague Grammar. Universal algebraic 
methods remained close to his heart ever since.

All these ideas came together in Theo's landmark dissertation "Foundations 
and Applications of Montague Grammar" (1983), with, in the jargon of the 
times, Peter van Emde Boas as a promotor, Renate Bartsch as a copromotor, 
and Johan van Benthem as a coreferent, together representing the various 
perspectives coming together in this work.

In the decades since, Theo Janssen became an acknowledged international 
expert on the topic of compositionality, who published widely on a range 
of topics, from the historical origins, where he discovered that the 
principle is wrongly attributed to Frege (a result he even presented in 
the Fregean heartland of the then German Democratic Republic) to the 
foundations of machine translation. He went on to publish many further 
insights into the mathematical foundations and linguistic applications of 
the topic. All these come together in his chapter on compositionality in 
the "Handbook of Logic and Language" (1997), which shows his mastery of 
the subject and his international recognition. In later years, one of his 
main interests were the challenges posed to compositionality by the 
game-theoretic semantics of Hintikka and others, and the proper 
understanding of the accompanying "information-friendly logics". His 
qualities of painstaking analysis, clarity of thought and presentation, 
and fearless standing up for his critical points, concerning curious 
variable-binding features of existing independence-friendly systems, were 
evident throughout.

In addition to his solid record in research and interdisciplinary 
bridge-building, Theo was also a dedicated and well-respected teacher for 
many generations of students, an active supervisor of master's and PhD 
students, an advocate and implementer of curriculum reform whenever the 
time was ripe, and, and in recent years also, of outreach to high schools.

The ILLC has lost one of its founding members who was there right from the 
initial period in the 1970s, helping build the interface of logic, 
language and computation, and exemplifying a harmonious combination of 
interdisciplinary spirit and mathematical standards. Moreover, Theo 
Janssen exhibited personal qualities that were plain for all to see: he 
thought deeply, he could be critical without social sugaring to anyone 
whenever called for --- but at the same time, he was a trusted team 
player, deeply loyal to his colleagues and his broader environment.

Theo will be missed by his colleagues and students. Our thoughts go out to 
his daughters Marlieke and Jorien.

Sonja Smets, director ILLC
Johan van Benthem
Peter van Emde Boas
Frank Veltman

(11 December 2018)
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