The dialogue is carried on by three interlocutors, two of whom adduce the scientific reasons for the double motion of the earth, while the third honestly tries to defend the opinions of the Aristotelian school with all the scientific means at his disposal, and as these did not suffice, with the arts of sophistry also. If he has but little success, the fault lies with the cause he advocates. Galileo gave to the defenders of the Copernican system the names of his two famous pupils and friends, neither of them then living, Filipo Salviati, of Florence, and Giovan Francesco Sagredo, senator of Venice, thereby erecting a better monument to them than he could have done in marble. Salviati is the special advocate of the Copernican theory. Sagredo takes the part of an educated layman, intelligent, impartial, and desirous to learn. The advocate of the Ptolemaic system was called briefly Simplicius, a pseudonym over which the learned have often puzzled their heads. Did he give this name of simpleton satirically to the champion of the ancient system, or was it merely an allusion to Simplicius, the commentator of Aristotle, as Galileo stated in his “Avviso al lettore?” The selection of this name is characteristic of the ambiguous attitude which the author maintains in his “Dialogues.” The sarcastic vein is obvious throughout, but is ingeniously concealed behind a mask intended to inspire confidence. Salviati conducts the arguments for the Copernican theory with such convincing force and clearness, and annihilates so completely all the objections of the unfortunate Simplicius, that no unbiassed reader can fail to perceive the scientific superiority of the modern theory to the old. And as Galileo conscientiously puts in the mouth of the Peripatetic philosopher every possible argument in favour of the Aristotelian cause, as well as the objections to the other side, the total defeat of its advocate is a victory all the more brilliant for the immortal Canon of Frauenburg.