Jack Carroll is one of the founder of HCI. I think he is the best theorist in this area. Below is the introduction from SIGCHI when Jack won the Lifetime Achievement Award in 2003:
"Jack is one of the pioneering thought leaders developing the intellectual foundations for HCI. His work, which ranges over philosophy, cognitive science, social science, systems theory, and design theory, is a creative integration of theory and practical methods, such as his work on scenario-based design methods. His work on the Blacksburg Electronic Village is one of the most successful and longest-running community participatory design experiments. Jack is probably the most prolific researcher in HCI - in addition to well over 300 papers, he has written and edited 14 influential books, including the just-published "HCI Models, Theories, and Frameworks". Jack's multitude of contributions to our understanding of the nature and practice of HCI is profound."
Jack's influences are profound and his contributions are versatile. He proposed minimalism in information and instruction design in 1980s, which has been accepted as common knowledge now in this area. Jack also proposed senario-based design in 1990's, which has been widely accepted in application and interface design. In early 1980's when users could first know personal computer, Jack conducted a series of studies revealing that users are active and creative adventurers in learning computer. He designed instructions such as "minimal manual" and and applications such as "training wheel" to help learning. He proposed task-artifact framework in HCI. He raised a very famous and influencing debate with other two big guys Newell and Card (fathers of GOMS) on the "soft science" vs. "hard science". I think the time tells he was right. He also had a lot of establishing and penetrating work in usability engineering, design rationale, participatory design, and community informatics.
Similar with his versatile contributions, Jack's background includes mathematics, information sciences (Lehigh Univ.), psychology (Collumbia Univ.), and linguistics (MIT). He worked with the famous linguist and philosopher Noam Chomsky for several years, and said the only thing he was sure there was that his interest did not lie in linguistics. Actually nobody respect Chomsky as he does. And actually he published a linguistic psychological paper in Science when he was a PhD student, which pointed out the radical problems and weak point of psychology. So he said he was not just a psychologist, but better than a psychologist.
Probably you can call Jack a psychologist, an information scientist, a linguist, or a philosopher. But he was never a computer scientist. But interestingly, he had been the head of the department of computer science in Virginia Tech for several years.
He is a very supportive academic advisor. Although he is so insightful, he is never directive to students. He said it is because when he led the legendary User Interface Institute at IBM he found other researchers usually took his temporal opinions or ideas for discussion as authoritative orders. He never push students, he just set himself as an example for the students. I like his style very much.
Jack has a lot of editorial services. He is one of the founder and the editor-in-chief of ACM Transactions on Computer- Human Interaction (TOCHI). He is in the advisory boards of International Journal of Human- Computer Studies, Human- Computer Interaction Series, Advances in Human- Computer Interaction Series, Handbook of User Interface Design, Handbook of Computer Science and Engineering, Human- Computer Interaction and Organizational Informatics, Handbook of Human-Computer Interaction, Kluwer Series in Human- Computer Interaction. He is the founding member of some of them. He is also in the editorial boards in almost 40 other famous journals in information science, cognitive science, psycholinguistics, psychology, communication, software, and the general journal Science. He teach classes such as IST 522 (HCI Theories, Models, and Frameworks) and IST 402 (Community Informatics).