A Princeton scholar will join Harvard's Philosophy Department on the tenured level next year as part of the University's effort to build up a core of specialists in moral and political philosophy, department members said yesterday.

Harvard has also extended a tenure offer to an Oxford University philosopher working in the same general field, a department member said. But Conant University Professor John Rawls said he was "Doubtful" Harvard would be able to convince Derek Parfit to leave England.

The new Harvard professor will be Thomas M. Scanlon, an ethics specialist who has written on such topics as philosophy of law, freedom of expression, rights, and notions of individual morality.

The appointment of the 43-year-old scholar is one of several that professors said the department is trying to make to build up its senior ranks, depicted over the last decade by retirements, unexpected departures, and several turn-downs of offers.

Even with the addition of Scanlon, there will be only nine tenured professors in the department, below a high of 13 in the late 1960s, said Professor of Philosophy Warren D. Goldfarb who was appointed two years ago.

In addition, professors said, this building effort is particularly important considering the age profile of the department which is unusually top heavy in relatively old scholars.

"It's a very excellent department but we really need people who will be here when the groups of people in their 50's and 60's retire." Department Chairman Robert Nozick said.

Nozick refused to comment on the offer to Purfit.

Professors in the department said they were especially interested in hiring scholars in the ethics and political philosophy area, because Harvard's two main practitioners at the moment--Rawls and Alford Professor of Natural Religion Roderick Firth--are nearing retirement age.

Scanlon's arrival will mark the culmination of a search for ethics specialists that has taken several years and has included at least one rejection of a tenure offer, from University of Michigan professor Allan F. Gibbard.

Harvard professors said they hoped to make more senior appointments not only in ethics, but also in topics like the history of knowledge and metaphysics Nozick refused to comment on how any other department searches were progressing.

The possibility of expansion in the department and the strength of the existing faculty were the principal reasons Scanlon gave yesterday for what he termed his "tough decision" to leave Princeton, where he has been since 1966, and since 1977 as a full professor Speaking from his New Jersey office, Scanlon, who received his Ph.D. at Harvard, sails. "There are not any concrete things I can point to, but it is an excellent department and I think I'll learn a lot from the people there."

The Princeton Philosophy Department chairman. Paul Benacerraf, said the Princeton had made a concerted effort to persuade Scanlon to stay, although he refused to detail the specifics.

"Harvard is extremely fortunate," he added "It is a very, very great regret to see him go--both as a personal friend and a colleague in the department."

Scanlon was described both by Benascerraf and Harvard professors as one of the leading think on working from the moderns "contraction" view of political philosophy originated by Rawis is his landmark book A Theory of Justice.