CAMBRIDGE is Artsy. Everybody knows. And Art is everywhere in Cambridge. Every little show on Mass. Ave. sports its own head posters, Night Watches and Van Gogh bridges along with Jewish holiday cards, plastic roses and scented pillows. But few bare walls are ugly enough to be insulted by a dollar's worth of commercialized psychedelia incongruously mushed with Great Oil Painting of the Western World.

Cambridge, however, is equally full of excellent prints and reproductions, though an important distinction should be made between the two. A print is a work which the artist himself designed to be printed in many copies and therefore planned in composition, color scheme, and impact with this in mind. A reproduction is a photographic copy of an original -- any original.

Original prints, even huge day-glo posters under black light are in their way more valuable than photographic reproductions of the Great Works of Fine Arts 13. With the poster you not only have the "picture" designed by the artist but the materials, the consistency and the total visual effect. With a paper edition of Rembrandt you hold only a deceitful shadow of the real object without the thick oil texture, the density, the depth that make the work great.

The irony is that it is not necessary to buy photographic reproductions to get "serious" work for one's serious study. Originals can be bought and rented for very reasonable prices and are amazingly exciting to have. Etchings and engravings by Picasso, Modigliani, Manet, Chagall, etc., original works, can be bought for less than twenty dollars at Retina Gallery, Fabrications, and at Gropper Gallery along Mass. Ave., just to name a few. These are small works and may come at the end of fifty or even a hundred impressions. But they are the right size for small rooms and very satisfying to own. When you own an original piece of art, you have the very object to which the artist put his hand.

PROBABLY the best selection of posters psychedelic is at the slick but very good Truc-in-the-Alley Poster Gallery, behind the Brattle Theater. Others are scattered all over groovy Cambridge, with some interesting ones at the Harvard Square Art Center near Inman Square on Mass. Ave., a few blocks from the Union.

Truc has by far the most exciting collection of posters around, ranging from fifty cents to fifty dollars. It has everything from homey "You don't have to be Jewish to Love Levy's" posters, and original Klee posters for his own exhibitions, to Paris street posters and Polish circus posters.

A few smaller stores carry special posters unique to them. The Gropper Gallery on Mass. Ave. near Radcliffe carries authentic World War II posters. Across the street a weird little Indian store, Kamala Devi, carries authentic demons and Buddhas and down in Brattle Square Zecropia sells wonderful Zodiac posters. Schoenhoff's, on Mass. Ave. between Plympton and Linden Streets, carries one of the best collections of reproductions of prints and paintings. Reproducing a lithograph or print is a relatively minor prostitution since the works were originally meant to be printed many times with paper and ink. Lacking the inscriptions and camp tone of many original posters, they often seem to be more serious work and are not quite as lively.

The Cambridge Art Association on Garden Street, right across from the Commander Hotel on Garden Street, rents and sells prints and paintings of local artists at very low prices and very good quality. There should be a student art gallery similar to it in Cambridge but since there isn't this one should get twice as much patronage. It is fun, not frightening, to pick out pictures according to your own tastes, instead of watered down versions of commonly accepted great masters.

It gets easier and easier to fool yourself. We all teach our eyes to lie to us about what they see. But ink isn't oil, paper isn't canvas, a dollar-fifty print isn't Rembrandt. Sometimes it seems worth it to care about what is real. And it will grow on you by January.