Career
He was Professor of Law at the University of Oslo Faculty of Law and editor-in-chief of Tidsskrift for Rettsvitenskap 1974–1999. He was also acting Supreme Court Justice in the Supreme Court of Norway. He grew up in Molde, where his father Odd Lassen was a lawyer
He obtained the examen artium in 1946.
After completing the Officer Cadet School, he went on to study law, obtaining the candidate.jur. degree in 1954. He was an assistant judge in Stavanger, before he was appointed research fellow at the University of Oslo in 1957.
He was appointed Lecturer in 1961, Reader in 1971 and Professor in 1990. He was also chairman of the board of Gyldendal Norsk Forlag, Norway"s largest publishing company.