The plan of the work, as indicated by the word dialogue, is that of an argument between supporters of the two systems. There are, however, three interlocutors instead of two, as it seemed expedient to provide a third party to weigh the merits of the controversy provided by the two champions, and to represent what is now commonly called "The man in the street". Galileo gave the name of Simplicio, a noted commentator on Aristotle, to the personage who has to produce all the arguments against the new system, however foolish they may be. To the other characters he gave the names of two former friends of his own, Salviati of Florence, who speaks really for Galileo himself, and Sagredo of Venice, who is responsible for common-sense objections, and for the introduction of a lighter vein from time to time, but who, as is only natural, is very ready to be convinced by Salviati's arguments. The exigencies of the censorship prevented any of the most powerful arguments being pushed right home, as it was necessary at every critical point to emphasise the formality that the Copernican system was not to be regarded as true but only as a not impossible hypothesis. Some such display might be made by a skilled fencer against a hopelessly inferior opponent, who wears, however, a coat of adamant guaranteed to shatter the blade that touches it, so that no victory is to be allowed, every stroke being obliged to recoil.
Of the four days over which the dialogue is spread, the first combats the Aristotelian doctrine of perfect and unchangeable heavens by means of the evidence of new stars and sun-spots, and emphasises the similarity between the earth, moon, and planets, pointing to Jupiter with his attendant moons as a model of the solar system with its attendant planets. The chief point on the second day is the principle of the difference between common and relative motion. The objection to the earth's rotation,
