Joan of Arc has been inspiring playwrights ever since the celebrated ecclesiastical barbecue at Rouen in 1431. The latest and perhaps least expected of these is Jules Feiffer, whose "Knock Knock" is being given an affecting, amusing production at Baltimore's Center Stage. If you are of a whimsical nature and have not yet discovered this playhouse at 700 N. Calvert St., now through May 15 is a fine time to do it.

Sharing a cabin in the woods which has all modern conveniences, two reclusive old bachelors are given to philosophical probings on the what-is reality theme. There's a knock knock at the door "Who's there?" asks Cohn. "Joan." "Joan who?" "Joan of Arc" and, in full armor with banner and sword, enters the Joan of tradition.

Because "the sky which has always been up there is not up there anymore is no cause for holocaust." declares Joan. Heaven "is now simply a place to move to." The voices that accompany her advise that "people will never know it's a holocaust. They'll adapt themselves.

It's a lovely situation and one should not really be surprised that Feiffer has found it. for in drawings and words he is ever attracted by the battle between practicality and idealism. His dippy. zany world is still a shambles. Unexpected visitors insert themselves. including a messenger who asks nosey questions, a poker player tricked by haphazard rules and a judge given to tangents. as well as a Joan condemned to housewifery and "the trick of getting the roast and the beans and the cauliflower and the potatoes all done at the same time.?

But Feiffer, letting Joan rise to the ceiling, gives her summation which a whollybeguiling young actress, Bess Armstrong, delivers deliciously:

"However lost you need to be found. however found. you need to change. however changed. you need simplicity. however simple, not too simple or oversimple . . . self-sufficent but not alienated, not despairing, not sneering. not cynical, not clinical, not dead unless you are dead and even then make the most of it."

The play had a curious 1976 New York experience. Produced Off-Broadway, it was up-graded for bigger things with a new director, Jose Quintero. and A flashier cast, after which it withered.

Baltimore's director John Fienry Davis was assistant director for the original and has staged it awarely, with considerable assist from scenic designer Charles Cosler. Robert Pastene is dry as Cohn. Herman O. Arbeit juicy as his pal and Edmond Genest does all sorts of vocal and visual trave and perky, is worthy of Shaw's Joan, completing a fine cast.