Galileo's thought experiment involving a smoothly sailing ship illustrated which foundational concept regarding movement?
Answer
Objects retain their motion unless acted upon by an external force.
This concept, central to his explanation for not feeling the Earth move, laid groundwork for later formulations of the laws of motion.

Related Questions
What was the most powerful, direct evidence Galileo lacked that would have definitively proved Earth’s orbit around the Sun?What astronomical discovery provided Galileo with a small, working model contradicting the idea that everything orbits Earth?In the traditional Earth-centered model, why would Venus almost always appear in a crescent phase to Earth observers?What observation about the Sun supported the idea that the heavens were not perfect and unchanging spheres?How did Galileo explain why people did not feel the rapid motion of a supposedly moving Earth?Galileo's thought experiment involving a smoothly sailing ship illustrated which foundational concept regarding movement?What action did the Catholic Church officially take regarding the proposition that the Earth moves and the Sun is the center in 1616?Which attempt by Galileo to support Copernicus is modern science considered a significant scientific error?What fundamental assumption about the cosmos was immediately undermined by the discovery of the moons orbiting Jupiter?What was the direct consequence for Galileo after publishing his comparative work *Dialogue Concerning the Two Chief World Systems*?