The works, which we have enumerated, consider the art of rhetoric in different points of view, and thus receive from each other mutual support and illustration, while they prevent the tediousness which might else arise, if they were moulded into one systematic treatise on the general subject. Three are in the form of dialogue; the rest are written in his own person. In all, except perhaps the _Orator_, he professes to have availed himself of the principles of the Aristotelic and Isocratean schools, selecting what was best in each of them, and, as occasion might offer, adding remarks and precepts of his own.[203] The subject of Oratory is considered in three distinct lights;[204] with reference to the case, the speaker, and the speech. The case, as respects its nature, is definite or indefinite; with reference to the hearer, it is judicial, deliberative, or descriptive; as regards the opponent, the division is fourfold--according as the fact, its nature, its quality, or its propriety is called in question. The art of the speaker is directed to five points: the discovery of persuasives (whether ethical, pathetical, or argumentative), arrangement, diction, memory, delivery. And the speech itself consists of six parts: introduction, statement of the case, division of the subject, proof, refutation, and conclusion. His treatises _De Inventione_ and _Topica_, the first and nearly the last of his compositions, are both on the invention of arguments, which he regards, with Aristotle, as the very foundation of the art; though he elsewhere confines the term eloquence, according to its derivation, to denote excellence of diction and delivery, to the exclusion of argumentative skill.[205] The former of these works was written at the age of twenty, and seems originally to have consisted of four books, of which but two remain.[206] In the first of these he considers rhetorical invention generally, supplies commonplaces for the six parts of an oration promiscuously, and gives a full analysis of the two forms of argument, syllogism and induction. In the second book he applies these rules particularly to the three subject-matters of rhetoric, the deliberative, the judicial, and the descriptive, dwelling principally on the judicial, as affording the most ample field for discussion. This treatise seems for the most part compiled from the writings of Aristotle, Isocrates, and Hermagoras;[207] and as such he alludes to it in the opening of his _De Oratore_ as deficient in the experience and judgment which nothing but time and practice can impart. Still it is an entertaining, nay, useful work; remarkable, even among Cicero's writings, for its uniform good sense, and less familiar to the scholar only because the greater part has been superseded by the compositions of his riper years.