The study of Cicero's philosophical works is invaluable, in order to understand the minds of those who came after him. Not only all Roman philosophy after his time, but a great part of that of the Middle Ages, was Greek philosophy filtered through Latin, and mainly founded on that of Cicero. Among his works on speculative philosophy are "The Academics, or a history and defense of the belief of the new Academy;" "Dialogues on the Supreme Good, the end of all moral action;" "The Tusculan Disputations," containing five treatises on the fear of death, the endurance of pain, power of wisdom over sorrow, the morbid passions, and the relation of virtue to happiness. His moral philosophy comprehends the "Duties," a stoical treatise on moral obligations, and the unequaled little essays on "Friendship and Old Age." His political works are "The Republic" and "The Law;" but these remains are fragmentary. The extent of Cicero's correspondence is almost incredible. Even those epistles which remain number more than eight hundred. In them we find the eloquence of the heart, not of the rhetorical school. They are models of pure Latinity, elegant without stiffness, the natural outpourings of a mind which could not give birth to an ungraceful idea. In his letters to Atticus he lays bare the secret of his heart; he trusts his life in his hands; he is not only his friend but his confidant, his second self. In the letters of Cicero we have the description of the period of Roman history, and the portrait of the inner life of Roman society in his day. 8. HISTORY.--In their historical literature the Romans exhibited a faithful transcript of their mind and character. History at once gratified their patriotism, and its investigations were in accordance with their love of the real and the practical. In this department, they were enabled to emulate the Greeks and to be their rivals, and sometimes their superiors. The elegant simplicity of Caesar is as attractive as that of Herodotus; none of the Greek historians surpasses Livy in talent for the picturesque and in the charm with which he invests his spirited and living stories; while for condensation of thought, terseness of expression, and political and philosophical acumen, Tacitus is not inferior to Thucydides. The catalogue of Roman historians contains many writers whose works are lost; such as L. Lucretius, the friend and correspondent of Cicero, L. Lucullus, the illustrious conqueror of Mithridates, and Cornelius Nepos, of whom only one work was preserved, the "Lives of Eminent Generals." The authenticity of this work is, however, disputed. But at the head of this department, as the great representatives of Roman history, stand Julius Caesar, Sallust, Livy, and Tacitus, all of whom, except the last, belong to the Augustan age.