Abstract
In his letter to Guidobardo Del Monte in 1602, Galileo Galilei claims that he derives the law of conjugate chords, Prop. 36 in Two New Sciences III. M. Yoshinaka, W. L. Wisan and S. Ito suggest that Galileo obtains the law of conjugate chords from the mean proportional theorem just like in Two New Sciences. In this paper it is shown that the law of conjugate chords can be derived from the theorem of De Motu. The historical significance of the laws of chords and conjugate chords is discussed and it is maintained that these laws are the link that joined old Aristotelian to new Newtonian dynamics.
