A shelf of books has been written upon the Greek sources of Cicero’s ideas, and if one were to discuss the manner in which Cicero’s own experiences modified those ideas before he accepted them for his own use one would ask for a second shelf of at least equal length. Cicero’s political works, like the _De Republica_ and the _De Legibus_ were written after an extensive perusal of Greek political masterpieces, but they are not, like many of the philosophical essays, paraphrases. The author betrays the fact that he has been in politics for a long time, that he has in fact been a party leader and has held the highest offices of state. He does deference abundantly to Plato, Polybius, and Panaetius for good suggestions, but it is an experienced Roman statesman who has the last word on every issue at stake. Cicero’s various political works are not all in agreement with each other nor with the utterances upon the same themes found in his letters and orations, nor do his political acts follow an unbending course. He lived in fact through a long period of revolutionary changes in politics, when consistency through a life-time would have betokened either inability to learn or stubborn intransigence. Drumann set him down as a turncoat, a judgment which Mommsen reiterated in a great variety of phrases. Heinze, in a mistaken attempt to rescue Cicero’s reputation, tried to prove that he had been a fairly consistent conservative through life. Zielinski, on the other hand, endeavored to show that Cicero’s theories could be traced to his reading, and that a search in these sources would explain Cicero’s somewhat wavering course. All these views seem to me to emerge from cloisters that are very far removed from the kind of democratic politics that Cicero lived through, a kind of politics not entirely unfamiliar to some of us from daily observation. Drumann and Mommsen wrote in an atmosphere where firm and consistent loyalty to the existing régime was expected of all gentlemen and where firm independence and detachment were taken as marks of vacillation; and even Heinze’s apology breathes some of the same spirit.[1] As for Cicero’s dependence upon the theories of his predecessors, it must be admitted that no Roman knew them better or received more from them. But professors who delve in books all their lives are apt to over-estimate the effect of written theory and of tralatician ideas, and to under-estimate the momentum of facts that compel practical men to take quick and unpremeditated action. Very often Cicero saw the value of an idea in Plato or Panaetius only after an experience of his own had thrown him pell-mell upon the realities that disclosed the meaning of their abstract ideas. I wish here very briefly to outline his changes in political thought against the background of his experience and his reading.