Perhaps the most significant change in the transition from Aristotelian or medieval science to classical or Newtonian mechanics is the change from the Aristotelian view of ’’Omne quod movetur ab alio movetur’’ (’’All that is moved is moved by something else’’) to Newton’s first law or the principle of inertia that ’’Every body continues in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed on it’’ [I. Newton,Principia(Univ. of California Press, Berkeley, 1960), Motte’s translation revised by Cajori, p. 13]. In this paper we shall show that this is not a sudden change but rather that it has a long and detailed history in the medieval critiques of Aristotle’s physics. The treatment of the problems of motion in a void, projectile motion, falling bodies, and the inclined plane by medieval scientists makes this transition quite understandable.