The Institute for Natural Language Processing at the University of Stuttgart commemorates its founder
Prof. Dr. Christian Rohrer,
who passed away in Meersburg on July 29, 2022, at the age of 84. The computational linguistics research community loses one of its greatest pioneers. Without his visionary strategic actions over many decades, the multifaceted research landscape between linguistics and computer science, characterized by a strong spirit of cooperation, would certainly not have developed in the same way.
Christian Rohrer, born in 1938, studied Romance languages and literature in Geneva, Tübingen, and Paris in the early 1960s and received his doctorate in Tübingen in 1965 with a dissertation on word composition in modern French. A postdoctoral stay took him to the University of Texas at Austin in 1965-1966, after which Rohrer earned his Habilitation in Tübingen with a thesis on functional linguistics and transformation grammar (published in 1971). In 1969, in his early 30s, he was appointed full professor in the Romance division of the Institute for Linguistics at the University of Stuttgart.
Early on, he recognized the potential that precisely specified formalisms and computer-implemented algorithms bear for the empirical validation of models of grammatical competence and meaning construction. Thus, already in 1971, shortly after his appointment in Stuttgart, he successfully applied for funding by the German Research Foundation for a project on formal semantics that targeted current key questions of international research and comprised three researcher positions — a unique set-up in the German research landscape at the time.
Rohrer's own contributions to formal semantics of French and German and their computational implementation, as well as his research on Machine Translation led to the remarkable situation in 1986 that he, as the head of an institute for language studies, prevailed in an external application for a chair in computational linguistics. In the course of the negotiations with the University of Stuttgart resulting from the external offer, Rohrer’s request was granted to establish a new Institute for Natural Language Processing comprising his own chair, a chair for Formal Logic and one for Experimental Phonetics (later on, a fourth chair for Theoretical Computational Linguistics was added).
Rohrer stayed loyal to this institute, the "IMS", until his retirement in 2006 and far beyond, continuing to shape the development of the research landscape between linguistics, computational linguistics and computer science through collaborative projects and international cooperations. Particular mention should be made of the Collaborative Research Center 340 "Linguistic Foundations for Computational Linguistics", run jointly between institutes in Stuttgart and Tübingen, which Rohrer chaired as spokesperson for the full duration of twelve years 1989-2000, and the international Parallel Grammar Development Project ParGram, starting out in 1994, which has been dedicated to broad-coverage grammar development across languages using the Lexical Functional Grammar (LFG) formalism. The ParGram founding consortium (soon to be joined by many sites working on additional languages) consisted of the Xerox Palo Alto Research Center, the Xerox Research Center Europe in Grenoble, and Rohrer's group at IMS, for an English, French, and German LFG grammar, respectively.
Beyond his specific achievements in research and in triggering institutional developments, Rohrer shaped several generations of linguists and computational linguists in their scholarly self-understanding. A basic element of this was the insight that, with the increasing degree of specialization in research sub-disciplines that had developed by the middle of the 20th century, further advances in our understanding of the strongly interleaved cognitive abilities underlying the human language capacity are best achieved through cross-disciplinary collaboration. This requires for collaborators to enter an open and respectful dialogue that does justice to different methodological traditions, to carefully establish interface representations and to share resources such as annotated corpora and analytical tools. Rohrer devised a concept for the IMS that approaches the complexity of language as an object of study by relying on a pluralism of methodological approaches; wherever possible, the expertise from different sub-disciplines should be combined. By implementing this concept in research and teaching at IMS, it was absorbed by generations of computational linguists.
Everyone who met Rohrer knows that he embodied the conciliatory spirit that is essential for promoting such an interdisciplinary approach like hardly anyone else — coupled with an incredible enthusiasm for cracking challenges regarding language and language processing. Not taking one's own approach to a problem to be absolute is crucial for transitioning from a collection of well-understood solutions to isolated subproblems to a more comprehensive model of the human language competence and of language processing. Such a comprehensive model then also allows researchers to test on an empirical basis precisely how partial solutions interact, which is what Rohrer always saw as a key goal. Ideal test cases for checking integrated models were sentence constructions which one would simply come across in real language use. The team members remember how often Rohrer called an ad hoc project meeting (by phoning them up in their offices) after typing a sentence he had read in a novel or in the daily newspaper into the LFG system and observing surprising effects resulting from interaction across different implemented analyses! (Needless to say that Rohrer was still up for this type of discussion as an emeritus professor when he biked to the IMS on a regular basis for many, many years.)
Part of the recipe for success was certainly also that for Rohrer it was understood that all dialogue across field boundaries is carried out on an equal footing — following the conviction that every exchange is a give and take. Some team members could easily assess the complexity class of a processing algorithm, while others understood the subtleties of the interplay between two linguistic phenomena and could quickly construct a suitable paraphrase to narrow down the causes of a prediction error. Moreover, Rohrer always conveyed that status hierarchy played no role in the dialogue, but only scientific interest in the subject matter. What the students had to say received the same attention as the word of an established authority. Respectful interaction independent of hierarchies was highly formative for the atmosphere established in IMS research and teaching — especially since the other professors shared this philosophy.
In the same vein, Rohrer's promotion and encouragement of early-career researchers, e.g. by assigning them competencies for responsible, independent action (an aspect that research funding agencies are currently again trying to foster in the academic system) was a matter of course over all the years, and it certainly contributed to the fact that a considerable number of IMS students, PhDs and postdocs went on to take over responsible positions inside and outside of academia, in Germany and abroad.
And last but not least, the emphasis on respectful interaction did not end in scientific dialogue. Rohrer for instance always found the right, appreciative tone in exchanges with the university administration as well — an aspect that should not be underestimated for the functioning of a medium-sized institute with many international connections.
The IMS and the research community in linguistics and computational linguistics owe Christian Rohrer an inestimable debt of gratitude. Due to his illness, it was unfortunately no longer possible for him to maintain an intensive scientific exchange in recent years. Now the news of his death makes us aware of the irretrievability of enthusiastic discussions with him on language processing and fills us with sadness, but at the same time with deep gratitude for all that he achieved for himself and the research community in a fulfilled life as a researcher.
(This obituary was written by Jonas Kuhn on behalf of the IMS.)