While the concept of inertia had been hinted at by earlier thinkers like Aristotle, Galileo was the first to formulate it as a clear and distinct principle through his experiments with rolling balls on inclined planes. He observed that objects in motion tend to stay in motion unless acted upon by an external force.

However, it was Isaac Newton who later refined and formalized Galileo's ideas into his famous First Law of Motion, also known as the Law of Inertia. Newton's law states:

> An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Therefore, while Galileo is credited with the initial discovery, Newton's work solidified the theory of inertia and gave it a more precise mathematical form.