Robert Ashley has been prompting the question since the 1960s when, as a director of the University of Michigan's Cooperative Studio for Electronic Music and a participant in the vigorously avant-garde ONCE Group, he began affixing the label ''opera'' to compositions that bear scant resemblance to anything in the operatic tradition. Significantly, Ashley's operas contain almost no singing and precious little in the way of plot. Although his operas are dense with words, the words tend only to be spoken, usually by Ashley himself in his dual role of narrator/principal performer. It may happen, in an Ashley opera, that a guy gets stabbed in the back, but the audience member will neither see the deed nor hear it described with music. He will only hear about it - sort of - in loose phrases cushioned on a soft ''bed'' of accompanying sound. This does not seem, on first glance, to be the stuff of opera.

Yet lately it has been the basic substance of a great many musical-theatrical works for which there is no other convenient tag. And it is certainly the substance of what Ashley - the seminal figure in all this - chooses to call opera. Ashley's impressively long list of operas includes ''In Memoriam: Kit Carson'' (1963), ''That Morning Thing'' (1967), ''Music With Roots in the Aether'' (1967), ''Atalanta'' (1982), ''Scenes on the Raft of Babel'' (1985), ''el./Aficionado'' (1987) and ''Now Eleanor's Idea'' (1988). Its central work, however, is ''Perfect Lives,'' a project that occupied him from 1977 to 1983 and which this month has been gloriously published in two very different formats.

The audio portions of ''Perfect Lives'' were released in dribbles - on the maverick label called Lovely Music Ltd. - as the sections were finished, and in the early '80s Ashley and his partners performed excerpts at festivals around the world. But these installments, intriguing though they were, amounted to mere samples. ''Perfect Lives'' was designed for television, not for recordings or for the stage, and it was always Ashley's hope that a television audience would be able to swallow it more or less whole. Commissioned by New York's Kitchen Center for Video, Music and Dance and produced by John Sanborn for England's Channel Four, the final version of ''Perfect Lives'' was completed in 1984. The seven half-hour episodes have been telecast several times in England and elsewhere in Europe, but never in this country. For several years, however, the videotapes have been available from Lovely Music Ltd. (the address is 10 Beach Street, New York, N.Y. 10013), and they make for fascinating viewing.

Sanborn's imagery is not synchronized in an obvious way with Ashley's score, yet invariably it follows the same course; one watches just as one listens, transfixed, caught up not so much with the ''beauty'' of it all as with the ineffably logical interplay of myriad elements. While it is most definitely a multimedia piece, ''Perfect Lives'' is not a Gesamtkunstwerk in the Wagnerian sense. For Wagner, the ''complete art work'' was one in which almost all the components sprang from a single creative mind. Ashley is no less concerned with completeness, but he is quite content to assign huge amounts of material to collaborators. In a television opera, one would expect the producer to have a relatively free hand, and that was indeed how Sanborn - with his crew of video editors and computer animation experts - approached the visual side of ''Perfect Lives.'' In any kind of opera, one would not expect such freedom to be granted to assisting musicians. Yet hardly a note of ''Perfect Lives'' comes directly from Ashley. The backdrops of electronic sound are credited to Peter Gordon; the occasional drum tracks belong to David Van Tieghem; the jazzy instrumental music that runs ceaselessly though the opera is the work of a virtuoso keyboard improviser who calls himself ''Blue'' Gene Tyranny. Even the audio mix - so crucial to a soundtrack in which a soft-spoken narration hovers just at the edge of audibility - was farmed out. The operaphile is entitled to wonder: What did the composer of ''Perfect Lives'' actually compose?

In the first place, Ashley came up with the concept for an opera that would reflect on ''our consciousness as Americans'' by depicting a slice of life in the Midwest (''Perfect Lives,'' which to an extent is about the idea of flatness, is set in Galesburg, Ill.). Secondly, he wrote and recited the text, a smooth stream of words whose meanings are often vague but whose lilt and cadence have as much to do with the overall tone of ''Perfect Lives'' as does any of the more conventional-sounding music. Perhaps most important, Ashley composed the ''rhythm templates'' on which everyth ing in the opera apparently is based.

Don't try to find ''rhythm template'' in a music dictionary. It's a term Ashley coined to describe the methods by which he determines large- and small-scale dimensions that exist in his works. Although the procedure has to do essentially with the passage of time, it can be translated into visual and literary equivalents. I had heard about Ashley's ''rhythm templates'' before, and I suppose earlier in the month I was once again subconsciously measuring them with my body clock as I listened to the whole of ''Perfect Lives'' in the compact-disc edition just issued by Lovely Music (LCD 4917.3). But it was not until I read the essay by Ashley that accompanies the long-awaited book version of the libretto (Archer Fields Press; $35) that I realized how broad the scope - and how potent the force - of such ''templates'' can be.

In one way or another, Ashley says, the rhythm of camera shots in ''Perfect Lives'' holds to precisely the same patterns that govern the rhythm of his vocal inflections and the harmonic rhythm of the piano playing. But the patterns also control proportions in the visual images: the ratio between the sky and the ground areas in a landscape, for example, or the size of a face relative to the entire TV screen. They control, too, the text - not just the pace of its delivery but the length of the lines, the placement of assonance and internal rhymes, even the subject matter. When I first made the acquaintance of ''Perfect Lives,'' through the piecemeal recordings that began to come out in the late '70s, I could absorb it only through my ears. When I encountered it in a quasi-concert situation for the first time, at the 1982 New Music America Festival in Chicago, listening conditions were so poor that in effect I absorbed it only with my eyes. With the videotape, of course, one gets a much clearer picture, but my experience this past week - as I sat in silence and simply watched a long portion of the opera - only confirmed what I'd felt in Chicago: ''Perfect Lives'' is an opera that looks amazingly like it sounds.

Now that the libretto has finally been published between hard covers, devotees of ''Perfect Lives'' are discovering they can approach the opera from yet another angle. They can just read it, and perhaps draw from the text alone the same impressions they get from the video and from the music. In the bargain, they can savor to the fullest words that on the recordings they'll probably miss. Ashley argues that all speech is a form of singing, and certainly Ashley's speech - breathy, calm, a shade twangy - is rich in musical detail. It is such a gentle music, though, so repetitive and so smoothly blended with its accompaniment, that after a short while one tends to forget that it contains real words. Like litanies in the Catholic Church and slogans chanted at political rallies, the narration of ''Perfect Lives'' eventually becomes meaningful only in the musical sense. The narration is music, but it is also poetry, and it speaks quite eloquently for itself: