Paul Oskar Kristeller's interpretation of Renaissance humanism as a "characteristic phase in what may be called the rhetorical tradition in Western culture"  has exercised an enormous influence on Renaissance studies. What I wish to do here is to call attention a group of works that affected in one way or another Kristeller's thinking about the Renaissance in general and about Renaissance humanism in particular.
Paul Oskar Kristeller first enunciated his thesis on the nature and origins of Renaissance humanism five years after he came to America from Italy -- in a lecture, to be precise, at Connecticut College on 9 March 1944.  Among other points, Kristeller showed that the word humanist began as. a vernacular term in Italian school culture (i. e., humanista) and that it referred to the teacher of a specific set of subjects. Kristeller's friend Augusto Campana had independently and nearly contemporaneously made the same discovery.  But neither Campana nor anyone else had traced the origins of Renaissance humanism back to its medieval Italian rhetorical roots and the French rhetorical commentary tradition.  Kristeller's knowledge of the texts of the Italian Renaissance was extraordinary; but so too was that of Jakob Burckhardt, Georg Voigt, Remigio Sabbadini, Vittorio Rossi, Giuseppe Saitta, and Eugenio Garin, to mention only some of the major commentators on Renaissance humanism predating or contemporary to Kristeller in the mid-1940s. Kristeller was different because he brought to the task a distinct preparation that separated him from other commentators. There is no accounting for originality, but knowledge of Kristeller's training helps to explain elements of his theory.
1. HANS VON ARNIM'S DIO VON PRUSA
Kristeller received his doctorate from the University of Heidelberg in 1928, qualifying as an expert in ancient philosophy with a dissertation on Plotinus.  He then returned to Berlin to gain professional competence in classical philology, There he enjoyed the tuition of a breathtakingly distinguished group of teachers. He participated in the seminars of Eduard Norden and Werner Jaeger, and attended the lectures of Ulrich von Wilamowitz-Mollendorff, Paul Maas, Eduard Meyer, Wilhelm Schulze, Friedrich Solmsen, and Richard Walzer.  Norden had a large knowledge of classical rhetoric; indeed, his famous Die Antike Kunstprosa (1898) would not have been possible without such a knowledge. Jaeger had made his reputation by his work on Aristotle, but he too had an interest in the antique rhetorical tradition. 
During his years in Berlin, Kristeller wrote seminar papers on Cicero's Orator and his oration Pro Murena, and passed his state board examination with a thesis on Pericles's speech at the end of book 1 of Thucydides.  Therefore, it is inconceivable that he did not read -- at this time, if not earlier -- Hans von Arnim's Leben und Werke des Dio von Prusa, mit einer Einleitung: Sophistik, Rhetorik, Philosophie in ihrem Kampf um die Jugendbildung (The Life and Writings of Dio of Prusa, with an Introduction: Sophistic, Rhetoric, Philosophy in Their Battle over the Education of Youth). Dio "Chrysosrom" of Prusa was a notable figure of the Second Sophistic during the Roman Empire. Arnim's Dio von Prusa is the first authority on antique rhetoric Krisreller cited in his Martin Classical Lectures of 1954, the definitive statement of his theory on Renaissance humanism.  Arnim would have been well known to Kristeller, in any case, from his Stoicorum Veterum Fragmenta, which was then as now indispensable to studen ts of ancient philosophy What made Dio von Prusa especially valuable from the perspective of someone interested in philosophy was its lengthy first part, announced in the book's subtitle, where Arnim traced the competition between rhetoric and philosophy from the fifth century B.C. to Dio's time in the second century A.D.  Armin made the broad sweep of antiquity his focus rather than an individual period and showed that these two educational cultures more often than not peacebly co-existed over the centuries despite celebrated moments when they broke out in open warfare against each other. Arnim thus habituated his readers to viewing rhetoric as an essential intellectual and cultural strand running through antiquity. The themes of Arnim's book (dedicated to his teacher Wilamowitz-Mollendorff) had become virtual cliches for German classicists by the first decades of the twentieth century.  Kristeller came to work on the Renaissance with this understanding of classical culture long installed as an integ ral part of his erudition. He therefore had the historical perspective which allowed him to speak of humanism as a "characteristic phase in what may be called the rhetorical tradition in Western culture" and to remark that "since the rhetorician offers to speak and to write about everything and the philosopher tries to think about everything, they have always been rivals in their claim to provide a universal training of the mind."  The essential component in this preparation, I would contend, was not so much Kristeller's training in classical philosophy in Heidelberg and Marburg as his training in the classical rhetorical tradition in Berlin.
2. HARRY BRESSLAU'S URKUNDENLEHRE
Into his old age Kristeller could recite the names and dates of all the Carolingian monarchs. As an undergraduate he had tried his hand not only at philosophy, but also at mathematics and medieval history. The results in mathematics made for a humorous section of his "A Life of Learning" lecture at the 1990 meeting of the American Council of Learned Societies.  The results in medieval history were more significant. His teachers in medieval history were Karl Hampe and Friedrich Bathgen in Heidelberg and Edmund Ernst Stengel in Marburg.  In an unpublished memoir, Kristeller remarks:
I had attended the lectures and seminars of two excellent professors of Medieval History (who emphasized the methodology and also the auxiliary disciplines), Karl Hampe and Friedrich Bathgen. The seminars had for their text Bresslau's Urkundenlehre. ... During the Spring of 1926 I had also attended successfully a similar seminar by Stengel in Marburg . . .. 
To anyone in the first half of the twentieth century working on medieval documents, Harry Bresslau's Handbuch der Urkundenlehre fur Deutschland und Italien was something of a bible, and it remains to this very day an invaluable guide.  By his own account, Kristeller used the book in two seminars and had a third seminar in which he did similar work.  What one must understand is that Bresslau dedicated a fifty-page section of his book to medieval notaries in chanceries and civic life, and that he also extensively discussed the ars dictaminis in Italy from the eleventh century onward.  Kristeller once told me that he first encountered the ars dictaminis in the pages of Bresslau. This aspect of his training came to fruition some twenty years later as a key component in Kristeller's seminal article "Humanism and Scholasticism in the Italian Renaissance." For Kristeller the humanist movement arose in the field "of grammatical and rhetorical studies. The humanist continued the medieval tradition in thes e fields, as represented, for example, by the ars dictaminis and the ars arengandi"; and "moreover, as chancellors and as teachers, the humanists, far from representing a new class, were the professional heirs and successors of the medieval rhetoricians, the so-called dictatores ...."  Once he started to look for medieval antecedents to Renaisance humanism, perhaps Kristeller would have made the connection between the ars dictaminis and Renaissance humanism whether he had participated in the seminars of Hampe and Bathgen or not. But the fact remains that he possessed a technical expertise in medieval documents that was certainly rare, if not unique, for historians of classical philosophy and …