To reach the general reader of his Dialogue concerning the Two Chief World Systems (1632), Galileo constructed the arguments as a classical dialogue. It is a philosophical conversation among three positions represented by characters: Sagredo, the nobleman non-expert who wants to know about these questions; Simplicio, the Aristotelian philosopher; and Salviati who presents “our friend’s” (Galileo’s) views.

On Day One, Salviati defends the human understanding and the use of mathematical reasoning, and proceeds to criticize Aristotle’s physics and reveal its errors.

On Day Three, Salviati argues against the logical objections to the earth’s motion; during the course of his refutation, he points out that some of the argument is only possible because of the discoveries made through the use of the telescope.

Salviati acknowledges the difficulty of making a choice between systems and so presents and attacks the arguments of opponents to Copernicus, again invoking the use of the telescope.

Salviati claims, as a caution, that “I do not give these arguments the status of either conclusiveness or of inconclusiveness, since (as I have said before) my intention has not been to solve anything about this momentous question, but merely to set forth those physical and astronomical reasons which the two sides can give me to set forth. I leave to others the decision, which ultimately should not be ambiguous, since one of the arrangements must be true and the other false. Hence it is not possible within the bounds of human learning that the reasons adopted by the right side should be anything but clearly conclusive, and those opposed to them, vain and ineffective.” And yet he also continues to state that opponents of Copernicus misuse the Scriptures in false support.

Near the end of Day Three, Sagredo begins to admit he prefers the simplicity of the new system, and Simplicio proposes the physics of a moving earth as the sticking point that would refute Copernicus. The rest of the Dialogue then turns to physics.

Galileo concludes his arguments with the required legal neutrality as to which system is true, though he has been devastating against the opposition.

So what are the arguments? Galileo

• avoids Scripture, as warned

• shows the changeable nature of the heavens

• presents new physics of simple mathematical relations of objects and actions

• emphasizes substance & dimensions & dynamics

• unifies the earthly and heavenly realms with a physics of natural motion

• explains why Earth must have diurnal rotation, and why it cannot be detected

• explains why Earth must have annual heliocentric motion, which can be proved

• explains how diurnal and annual motions cause the tides, proving both must exist

• presents a comprehensive demolishing of Aristotelian mechanics and physics of motion

• dismisses the argument that God has the power to arrange things in ways no one can understand

This last item was provocative. In discussing the tides, Galileo has Simplicio argue that "Man cannot presume to know how the world really is, since God could have brought about the same effects in ways unimagined by humans. It is not proper to restrict God’s omnipotence.” Salviati responds, "Surely, God could have caused birds to fly with their bones made of solid gold, with their veins full of quicksilver, with their flesh heavier than lead, and with their wings exceedingly small. He did not, and that ought to show something. It is only in order to shield your ignorance that you put the Lord at every turn to the refuge of a miracle." Unfortunately, Simplicio's argument was one much favored by the Pope.