Giles of Rome analyzed the question of the division and definition of philosophy three times at the beginning of his philosophical career. He devoted to this subject the prologues of two of his Aristotle commentaries, Commentary on the Physics and Commentary on the Sophistical Refutations.Aegidius Romanus, Commentaria in octo libros Physicorum Aristotelis, (Venetiis, 1502); In Aristotelis De sophisticis elenchis commentum, (Venetiis, 1496–1497). The Prologue and I Book of Giles’s Commentary on the Physics dates to 1274; the Commentary on the Sophistical Refutations dates to 1275. See S. Donati, Studi per una cronologia delle opere di Egidio Romano, “Documenti e studi sulla tradizione filosofica medievale” (1990/1), pp. 1–111, pp. 42–48 (esp. n. 106). He then devoted a work exclusively to this subject, De partibus philosophiae essentialibus (De partibus [DPPE]).I provide an edition of De partibus philosophiae essentialibus in the appendix. De partibus belongs to the early period of Giles’s carrier. However, this dating is based on weak foundations, mostly relying on the formal and conceptual similarities between De partibus philosophiae essentialibus and De differentia rhetoricae, ethicae et politicae. See the ed. and intro. by G. Bruni, “The New Scholasticism” VI (1932), pp. 1–18. For the dating of Giles’s letter (ca. 1278), see p. 4. These associations are provoked by the old printed editions, whose editors usually joined both Giles’s works in their publications. Indeed, we have only one sure testimony enabling us to determine the terminus post quem of De partibus philosophiae essentialibus. It is reference made by Giles to his own In Sophisticis elenchis dated 1275. It is possible that De partibus philosophiae essentialibus was written soon after the Commentary on the Sophistical Refutations. The argument for dating De partibus philosophiae essentialibus to 1275 or 1276 rests on similarities between the two works. The last part of De partibus philosophiae essentialibus concerning logic (187–255) is virtually identical to passages from the Commentary on the Sophistical Refutations quoted in nn. 36 and 37. Because of its clear, systematic approach, this text will be the main object of my analysis. I shall, however, discuss material from the two prologues that demonstrates either the evolution of Giles’s thought from the two prologues to De partibus or the changes, corrections, or additions he introduced into the theory of science and philosophy formulated in De partibus.