Is the statement that two objects in free fall in a local gravitational field with different densities will hit the ground at the same time in a vacuum a law of motion as stated by Galileo or is it similar to something he stated, or just a corollary of the other laws? Is there any reference where he actually writes down laws of motion in the same way that Newton did in the Principia? Sorry if this is well-known, I don't really know the exact historical details before Newton.