Isaac Newton (1643-1727), professor of mathematics at Cambridge University, pondered the theories and observations of Copernicus, Brahe, Galileo, Kepler, Descartes, Huygens, Hooke, and others for over 20 years. Then he composed his masterpiece, the Principia, in Latin in just two years. This monumental three-volume work, filled with text and geometric illustrations, was published in 1687 by Newton’s benefactor, English astronomer Edmund Halley. Early in the Principia, Newton set forth three postulates which are commonly referred to as “Newton’s Laws of Motion.” 
Newton’s First Law of Motion (the law of ‘inertia’) states:
Newton defined inertia as the “innate force of matter…[its] power of resisting…” In this regard, it can be asserted that the inertia of a body (its power or tendency to resist change in its state of motion or of relative rest) is proportional to its mass. This concept is now generally called “inertial mass.”
Unlike Galileo, who described the orbital motions of the planets as inertial, Newton asserted that inertial motion must have constant direction of motion in a straight line as well as a constant magnitude (speed). In other words, inertial motion must maintain constant velocity, because “velocity” is defined as the speed (constant rate of motion) of an object in a particular direction (Figure 8).
Newton’s first law also tells us whether or not a net external force is acting upon a body. The reason is because: a) by Newton’s definition, “inertial” means “uniform motion in a straight-line;” therefore b) if a body (i.e. a planet) moves in a curved path, then according to Newton’s first law of motion, there must be net external “forces impressed upon it.” 
Kepler introduced the Latin word “inertia” (meaning “laziness”) to physics. Galileo described uniform motion as where the increments of distance, time and speed “repeat itself always in the same manner.” French philosopher Rene Descartes (1596 – 1650) first clearly described the phenomenon of inertia and described it as a “state” in his unpublished book, “Le Monde.” French scientist Pierre Gassendi (1592 – 1655) first published the law of inertia and tested it with experiments. Thus, Newton’s fully developed law of inertia was really a group effort.