"Every body constituted in a state of rest but naturally capable of motion will move when set at liberty only if it has a natural tendency toward some particular place; for if it were indifferent to all places it would remain at rest, having no more cause to move one way than another. Having such a tendency, it naturally follows that in its motion it will be continually accelerating."
[Circular motion] "being the motion that makes the moving body continually leave and continually arrive at the end, it alone can be essentially uniform. For acceleration occurs in a moving body when it is approaching the point towards which it has a tendency, and retardation occurs because of its reluctance to leave and go away from that point; and since in circular motion the moving body is continually going away from and approaching its natural terminus, the repulsion and the inclination are always of equal strength in it. This equality gives rise to a speed which is neither retarded nor accelerated; that is, a uniformity of motion."
quoted from Westfall, R. S. (1971) Force in Newton's Physics, the Science of Dynamics in the Seventeenth Century. Macdonald, London.