OPERA, says Paul Robinson in his new study, is an expression of the intellectual trends and concerns of its day. The statement seems obvious: few people are likely to bristle at the suggestion that art, even at its most abstract, comes most fully to life for those who have some feeling for the ideas that were in the air at the time it was conceived, and that opera, consisting as it does of music with an explicit dramatic component, should be doubly reflective of its era. But Robinson's approach to the question is intriguing enough to sustain interest even when his methods verge on overkill.

Instead of concentrating exclusively on opera libretti, where political and philosophical attitudes can be sniffed out in their least ambiguous form, Robinson turns to the scores to show how composers project their views using the rhetoric available to abstract music. This method is sound and promises to yield new insights, but Robinson sometimes gets carried away.

"I presume that I don't need to explain what I mean by 'opera,'" he states at the outset, but some readers may think otherwise when they turn to the table of contents and find the chapter on early Romantic opera given over to discussion of two Schubert song cycles.

Robinson offers the first of numerous justifications for trespassing among genres in his introduction: "The difference between an opera and a song cycle turns out to be less consequential than one might expect, since in the latter, too, music and ideas are united through the medium of language." Unless "opera" is to be accepted as a generic term covering all vocal music, this won't do.

Anyone who can suspend disbelief on the matter of definitions and read on will find plenty to inspire both admiration and exasperation. Robinson is at his best in the opening and closing chapters of the book, where he compares operas which differ profoundly in their intellectual outlook, although their dates of composition are relatively close. The first of these chapters, which contrasts Mozart's The Marriage of Figaro (1782) with Rossini's The Barber of Seville (1816), is especially interesting, and gets the book off to a good start.

Both operas are essentially comic; both take their libretti from plays by Beaumarchais which share common characters, yet from their opening bars the two works are worlds apart. Robinson traces the difference to the shift in attitude that transformed the European intellectual climate as the Enlightenment succumbed to various reactions against it. He sees Figaro as a supreme statement of the 18th-century concept of mind. The underlying idea of the opera, he says, is that of reasoned reconciliation; Mozart incorporates it into his complicated and highly rational structural and harmonic compositional procedures, focusing the idea with great emotional intensity in the final scene where the Countess Almaviva extends forgiveness to the unfaithful Count.

By the time Rossini began composing music for The Barber of Seville, a kind of prequel to Figaro, three decades later, Europe was in full and complex reaction against the previous century's enshrinement of Reason. One aspect of this revolt was distrust of intellect and idealistic political thinking. Robinson finds a musical analogy in Rossini's cynical treatment of Beaumarchais: "The Barber of Seville is the perfect operatic realization of this mood of intellectual and emotional retreat. It is relentlessly unserious, displays human viciousness in all conceivable guises, and refuses any kind of psychological or moral investment."

He makes his case in part by contrasting Mozart's and Rossini's treatments of the same chracters. Whereas Mozart's more complex characterization permits them to have their own minds and emotions in accordance with the ideals of the Enlightenment, Rossini prefers the methods of caricature, serving up travesty and irony "as a defense mechanism against the Enlightenment's dangerous humanism."

ROBINSON'S DISCUSSION of musical Romanticism, the other great reaction to the Enlightenment, is less graceful. Schubert and Berlioz are singled out as representatives of the lyric and heroic styles. Robinson says he settled on Schubert "because his subjectivity impresses me as more radical than that of any other great Romantic composer."

Now, this is the sort of statement that can set fists flying among excitable music students. But if these givens can be accepted, there is a good deal to savor in Robinson's discussion of Die Schoene Muellerin and Winterreise as musical expressions of the "unprecedented subjectivity of the Romantic movement." He sees the two cycles as roughly analogous to Blake's Songs of Innocence and Songs of Experience, respectively and shows how Schubert's music reflects these parallels with simple strophic songs predominating in his account of the experiences of the naive miller in the first cycle, and more highly evolved forms used to evoke those of the more sophisticated winter traveler of the latter work as he becomes increasingly more irrational on his endless journey.

More stridently argued and insistent, the studies of Berlioz The Trojans and Verdi's Don Carlo are also more far-fetched. "To state the matter baldly" Robinson writes, "The Trojans is a musical embodiment of the Hegelian idea of history." He supports this with evidence of Berlioz's all-out efforts to imbue the huge opera with a sense of inexorable forward motion: potent marches with gigantic scoring, rapid tempi, vocal writing, such as that for Aeneas, which gains in urgency from having been placed at the top of the singer's range. He also points out that erotic love, which suspends time and is the natural enemy of historical progress, brings the pulse of the opera way down in the fourth act, which recounts Aeneas' passion for Dido.

Although many listeners might think of Verdi as the Romantic opera composer par excellence, Robinson prefers to see him as the embodiment of Realpolitik, and once again he finds evidence for this in the score, this time that of Don Carlo. What he cites, however, are what he claims represent the rhetoric of political speech translated into musical terms. The high proportion of loud music, popular music effects and simple, easily memorizable tunes all point to an assertiveness in which he discerns political overtones. More convincing for this reader is Robinson's analysis of Verdi's technique of expressing his characters' political attitudes in the music he composes for them.

The comparson of Wagner's Die Meistersinger and Richard Strauss' Der Rosenkavalier with which Robinson ends Opera & Ideas neatly balances the book's opening dicussion. Here, again, the two operas have many features in common. Both treat similar situations of "romantic sacrifice," but, as with Mozart and Rossini, methods and viewpoints differ critically. Wagner uses the situation to explore 19th-century ideas about the social role of art whereas von Hofmannsthal's libretto for Der Rosenkavalier,and Strauss' treatment of it, are already completely 20th century in their psychologizing.

It is a shame that Robinson decided to leave his treatment of operatic modernism with Der Rosenkavalier. Each of the great 20th-century operas demonstrates his thesis intriguingly. A discussion of how, for example, Berg used the new musical language of Wozzeck to extract the latent Expressionism from Buechner's century-old play would have been an interesting extension, and may have been more convincing than some of the politicizing of Verdi's melodies and dynamics found in Opera & Ideas. But, despite some weak spots, Robinson's book permits its readers to explore new territory in works that have come to seem familiar, and most opera lovers will always be grateful for that kind of discovery.