Mass of a body is a quantitative or numerical measure of a body's inertia. In simple words, inertia is a measure of the tendency of an object at rest to stay at rest or of an object in motion to stay in motion. It is difficult to change the state of rest or uniform motion of a body of heavier mass and vice-versa. The mass of a body is the measure of its inertia of translational motion. Inertia is the property of a body due to which it opposes any change in its state. Thus, Newton explains his first law of motion based on the inertia of rest, the inertia of motion, and the inertia of direction. The law defines the force and states it as a factor, which can change the state of the object. A body continues to be in its state of rest or uniform motion in a straight line unless compelled by an external force to change its state. Newton's First Law of Motion is also known as Galileo's law of inertia. To be specific, the first law of motion defines the force qualitatively, the second law of motion gives a quantitative measure of the force, and the third law of motion states that a single isolated force doesn't exist. The three laws of motion describe the relationship between a body and the forces acting upon it, along with the body's motion corresponding to those forces.
The three laws of motion proposed by Newton are three physical laws, which together form the basis of classical mechanics.
While there are many theories on motion given by scholars and ancient scientists, Newton's Laws of Motion are the most popular. According to him, resistance (inertia) or friction affects a body in motion or at rest. Galileo Galilei takes a different angle to explain the laws of motion.
It is called Aristotle's Fallacy, and Newton's Laws come under Aristotle's Fallacy. According to Aristotle, a constant continuous force is required to keep a body in uniform motion which is not actually true.