The conversational partners of the Dialogue

Of the three people taking part in the Dialogue, Sagredo and Salviati bear the names of two friends of Galileo's who had already died by this time and whom he thereby wanted to make immortal. On the other hand, Simplicio, the third in the conversation, to whom Galileo assigns the task of defending the Earth as immobile and being at the centre, is a fictitious character.

Giovan Francesco Sagredo (1571–1620)

Sagredo belonged to one of the richest and most powerful families in Venice. Galileo lived in the sovereign territory for a while and had close contact with him. The Sagredo character in the Dialogue has his lively and brilliant personality resurrected once again: many allusions and episodes mentioned in the Dialogue come from the nobleman’s actual biography. Galileo has his purely fictional Dialogue, which lasts four days, take place in the Sagredo family’s Venetian palace.

Filippo Salviati (1582–1614)

Salviati came from a noble Florentine family that wielded great economic and political influence. He was a member of the Accademia della Crusca and from 1612, thanks to Galileo's influence, also a member of the Accademia dei Lincei. Galileo called him "an extremely witty nobleman" whom he "loved and cherished". Federico Cesi, President of the Accademia dei Lincei, called Salviati a "very distinguished and highly educated Florentine". In Galileo's fictional Dialogue, Salviati’s particular task is to represent the Copernican standpoint and – by picking up on the ancient doctrine of the Pythagoreans – to expound and defend Galileo's actual thoughts. For this purpose Galileo occasionally lets Salviati quote verbatim from his earlier works.

Simplicio

The third person in the conversation is deliberately enigmatic. His name was also chosen skilfully: Simplicio. Galileo implies that he is alluding to a real person, as with the other two people in the conversation, but this time he has deliberately chosen a pseudonym. He obviously wanted to arouse his readers' curiosity.

Nor was he afraid of playing games with his readers by of all things giving the peripatetic philosopher tasked with advocating Aristotle's teaching the name Simplicius. This name seems "fitting and appropriate" to him on account of his exceptional admiration for the comments of the identically named Greek philosopher Simplicius of Cilicia (6th century BCE). It was of course generally known that Simplicio also stood for "simpleton".