When President Reagan delivers a speech, Thor Sevcenko notes, nobody assumes he wrote it himself.

Conversely, says the professor of Byzantine History and Literature, "When an emperor writes a text in the 10th century, everyone thinks he is the author."

As a matter of fact, for more than 300 years, Sevcenko says, scholars have generally thought that a 10th-century manuscript chronicling the life of Basil, a ninth-century Byzantine emperor, was written by the ruler's grandson, the emperor Constantine Porphyrogenitus.

However, as Sevcenko begins putting on the finishing touches to 10 years worth of on-and-off editing and translating of that particular text, he adds that the conventional wisdom is apparently wrong.

The book--the only secular biography coming out of the Byzantine period--"purports to be the life of the grandfather by the grandson," says the 61-year-old scholar. "I purport it to be the life of the grandfather by a ghost-writer."

Byzantine Sherlock Holmes

One of the world's foremost authorities on the culture of the Byzantine Empire, which lasted from the fall of Rome (476) to the fall of Constantinople in 1452. Sevcenko has spent most of his adult life conducting such types of scholarly detective work.

In fact, for almost all of his adult life, Sevcenko has been digging through the labyrinthine texts and manuscripts of the age, starting with his coming of age in wartime Eastern Europe 40 years ago.

The only son of Ukrainian emigres, Sevcenko was born just outside Warsaw in 1922. After his graduation from a lyceum in 1939, the 17-year-old Sevcenko seized on a chance to leave that beleagured city and his job of selling books on the street.

He journeyed to Prague, where he lived out the war years in relative safety, earning his first doctorate in classical philology--the study of Greek and Latin linguistics and literary texts--in 1945.

"I was much better off than most of the people of my generation," remembers Sevcenko, speaking from his book-lined Widener study. "You have to have had a certain amount of luck and instinct for survival to survive through those years--even in a relatively sheltered existence."

Survivor

A true survivor, Sevcenko started a long trek west as Germany began to collapse, stop-pint first in Belgium, where in 1949 he earned a second doctorate, again in classical philology at the Catholic University of Louvain.

In Belgium, the budding university professor also became acquainted with the field of Byzantinology--introduced by a famous University of Brussels scholar, Henri Gregoire.

And ever since then, Sevcenko has added his long and varied academic career to this sparsely populated field, most recently as a tenured professor in the Classics Department completing his first decade at Harvard.