Although Roberto Rossellini was indisputably one of the most influential of European directors, the fact that his influence was founded upon a philosophical or conceptual attitude rather than upon the imitation of stylistic devices or thematic preoccupations has meant that Anglo-American film historians in particular tend to pay little more than lip service to his achievement, when not ignoring it altogether. Yet such indifference constitutes a major injustice not merely to a director who redefined the parameters of the cinema but to one of the supreme "documenters," in any art form, of the latter half of the 20th century.
Roberto Rossellini is one of those curious figures in world cinema who seems to generate more respect than admiration. As Adair accurately points out, the reverence typically paid to the director is little more than tokenism. Certainly, his realist aesthetic -- location shooting, natural light, non-professional actors, longer-duration takes -- has benefited filmmakers of sparse means the world over. He is remembered as a figure of undeniable historical significance without being thought of as the equal to directors of a similar influence (such as Griffith, Renoir, Welles, Bresson, Godard). He certainly lacks the devotion many hold for his best-known countrymen, from Fellini to Antonioni to Bertolucci, among others. However, I would submit that he is at once their superior and the greatest of all Italian directors, without even making any allowance for his firm place in film history. This is to say that his corpus speaks for itself, outside of history, as one of the most beautiful and inspired that the medium has yet seen.
Yet, again he is underestimated. The reason for this, it would seem to me, is that Rossellini's style is too often considered for its historical circumstances, its facility for assimilation, and even for what he expresses through this style (or less opaquely, his themes) rather than for what this style says about the director's viewpoint. Indeed, it is in an examination of its conceptual genesis that a unity emerges to Rossellini's corpus, marking him as one of the medium's best.
Specifically, it seems to me that the key to appreciating Rossellini rests in his epistemological ideas. For Rossellini, the truth is evident on the surface of things. From his early, immediately post-war films (Rome, Open City , Paisa , Germany, Year Zero ) to his Bergman cycle (including Stromboli , Voyage in Italy  and Fear ), his historical recreations (The Flowers of St. Francis  and The Rise to Power of Louis XIV  among others) and more proper documentaries (such as India ), what marks his entire body of mature work is a faith in humankind's perception of truth. Take the somewhat forgotten, small masterwork Fear: throughout the picture, Bergman's character attempts to conceal the truth of an adulterous affair. However, her body language belies the secret she is attempting to hide -- at one point her blackmailer asks rhetorically why she blushes, why her hands trembled and why she was so quick to give away her money. In other words, in spite of a shared Catholic faith with Robert Bresson, Rossellini departs from the Frenchmen (as well as from such directors as Michelangelo Antonioni, Wong Kar-wai, and even Eric Rohmer in Triple Agent ) in the his view that psychology is discernible in gesture.
As it has been claimed, this basic understanding of the comprehensibility of truth is not limited in fact to the psychologically-intensive Bergman cycle, but manifests itself throughout his ouevre. The post-war texts offer their morals within the context (another key theme in Rossellini) of a war-ravaged Europe. In seeing the condition of Germany in the 1948 film, one understands the despair of the child protagonist. Surface reality discloses truth. Similarly, The Flowers of St. Francis, perhaps his greatest work, shows (key theme number three) the basic rhythm of a long-since extinct life -- a life lived absent of any goals other than the service of the Lord -- which as such reveals Christianity's distinct understanding of time; less abstractly, India proposes that its presentation of Indian life is sufficient to reveal essential truths of the nation.
In this basic understanding of epistemology, I would argue, Rossellini shows himself to be an artist seeking a form to express his ideas, not simply the innovator of a technique born of historical circumstance. It is in this consistency of viewpoint, manifesting itself in his form, that the beauty of his work becomes clear. Rossellini's cinema deserves to be appreciated not only for its historical value, but indeed for the beauty of its expression of content. In this lies its grace.