I'm a little bit confused about the relationship between Galileo's Principle of Relativity and Newton's Laws. Indeed, as I understand, the Galilean Principle of Relativity is what Galileo presented with Salviatti's ship discussion. The discussion seems to lead to a simple idea: "if one performs mechanics experiments and makes observations at rest it will be the same as if it he were in uniform motion along a straight line". As a consequence, observing the phenomenon would not allow one to detect the movement since it wouldn't be able to tell the difference. Nowadays, however, the principle is stated as "the laws of mechanics are invariant in every inertial reference frame". This, however, requires an understanding of what one inertial reference frame is supposed to be. As far as I know, the idea of inertial reference frame is provided by Newton's laws. Indeed as I know one inertial reference frame is considered as one in which Newton's laws holds. But given this idea, Galileo's principle written as "the laws of mechanics are invariant in every inertial reference frame" seems meaningless: it should be like that by the definition of an inertial frame. I also know that Newton's Laws came just after Galileo, so his discussion was made before anyone knew about his laws of motion. In that case, first of all, how Galileo's discussion of Salviatti's ship relates to the statement that "the laws of mechanics are invariant in every inertial reference frame"? And also, how his principle relates to Newton's laws? Is it already included in the definition of an inertial frame?