For original authorities in reference to the matter of this chapter, read Diogenes Laertius' Lives of the Philosophers; the Writings of Plato and Aristotle; Cicero, De Nat., De Or., De Offic., De Div., De Fin., Tusc. Quaest.; Xenophon, Memorabilia; Boethius, De Idea Hist. Phil.; Lucretius. The great modern authorities are the Germans, and these are very numerous. Among the most famous writers on the history of philosophy, are Bruckner, Hegel, Brandis, I. G. Buhle, Tennemann, Ritter, Plessing, Schwegler, Hermann, Meiners, Stallbaum, and Speugel. The history of Ritter is well translated, and is always learned and suggestive. Tennemann, translated by Morell, is a good manual, brief, but clear. In connection with the writings of the Germans, the great work of Cousin should be consulted. The English historians of ancient philosophy are not so numerous as the Germans. The work of Enfield is based on Bruckner, or is rather an abridgment. Archer Butler's Lectures are suggestive and able, but discursive and vague, as is the History of Ancient Philosophy by Maurice. Grote has written learnedly on Socrates and the other great lights. Lewes' Biographical History of Philosophy has the merit of clearness, and is very interesting, but rather superficial. Henry has written a good epitome. See also Stanley's History of Philosophy, and the articles in Smith's Dictionary, on the leading ancient philosophers. Donaldson's continuation of Muller's History of the Lit. of Greece, is learned, and should be consulted with Thompson's Notes on Archer Butler. There are also fine articles in the Encyclopedias Britannica and Metropolitana. Schleirmacher, on Socrates, translated by Bishop Thirlwall. CHAPTER IX. SCIENTIFIC KNOWLEDGE AMONG THE ROMANS. [Sidenote: Wonders of modern science.] [Sidenote: Every great age distinguished for something never afterwards equaled.]