But the other portion of Plato's sum total of what is necessary to an Art of Rhetoric, is not open to the same objection. It involves no incompatible conditions: and we can say nothing against it, except that it requires a breadth and logical command of scientific data, far greater than there is the smallest chance of attaining. That Art is an assemblage of processes, directed to a definite end, and prescribed by rules which themselves rest upon scientific data--we find first announced in the works of Plato.[108] A vast amount of scientific research, both inductive and deductive, is here assumed as an indispensable foundation--and even as a portion--of what he calls the Art of Rhetoric: first, a science of psychology, complete both in its principles and details: next, an exhaustive catalogue and classification of the various modes of operative speech, with their respective impression upon each different class of minds. So prodigious a measure of scientific requirement has never yet been filled up: of course, therefore, no one has ever put together a body of precepts commensurate with it. Aristotle, following partially the large conceptions of his master, has given a comprehensive view of many among the theoretical postulates of Rhetoric; and has partially enumerated the varieties both of persuadable auditors, and of persuasive means available to the speaker for guiding them. Cicero, Dionysius of Halikarnassus, Quintilian, have furnished valuable contributions towards this last category of data, but not much towards the first: being all of them defective in breadth of psychological theory. Nor has Plato himself done anything to work out his conception in detail or to provide suitable rules for it. We read it only as an impressive sketch--a grand but unattainable _idéal_--"qualem nequeo monstrare et sentio tantum". [Footnote 108: I repeat the citation from the Phædrus, one of the most striking passages in Plato, p. 271 D. [Greek: e)/peidê\ lo/gou du/namis tugcha/nei psuchagôgi/a ou)=sa, to\n me/llonta r(êtoriko\n e)/sesthai a)na/gkê ei)de/nai psuchê\ o(/sa ei)/dê e)/chei. e)/stin ou)=n to/sa kai\ to/sa, kai\ toi=a kai\ toi=a; o(/then oi( me\n toioi/de, oi( de\ toioi/de gi/gnontai. tou/tôn de\ dê\ diê|rême/nôn, lo/gôn au)= to/sa kai\ to/sa e)/stin ei)/dê, toio/nde e(/kaston. oi( me\n ou)=n toioi/de u(po\ tô=n toiô=nde logôn dia\ tê/nde tê\n aiti/an e)s ta\ toia/de eu)peithei=s, oi( de\ toioi/de dia\ ta/de duspeithei=s], &c. Comp. p. 261 A. The relation of Art to Science is thus perspicuously stated by Mr. John Stuart Mill, in the concluding chapter of his System of Logic, Ratiocinative and Inductive (Book vi. ch. xii. § 2):