Galileo Galilei
Inertia

Galileo also concluded that objects retain their velocity in the absence of any impediments to their motion, thereby contradicting the generally accepted Aristotelian hypothesis that a body could only remain in so-called "violent", "unnatural", or "forced" motion so long as an agent of change (the "mover") continued to act on it. Philosophical ideas relating to inertia had been proposed by John Philoponus and Jean Buridan. Galileo stated:

Imagine any particle projected along a horizontal plane without friction; then we know, from what has been more fully explained in the preceding pages, that this particle will move along this same plane with a motion which is uniform and perpetual, provided the plane has no limits.

But the surface of the earth would be an instance of such a plane if all its unevenness could be removed. This was incorporated into Newton's laws of motion (first law), except for the direction of the motion: Newton's is straight, Galileo's is circular (for example, the planets' motion around the Sun, which according to him, and unlike Newton, takes place in absence of gravity). According to Dijksterhuis Galileo's conception of inertia as a tendency to persevere in circular motion is closely related to his Copernican conviction.
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