Newton assumed that "things moving in a straight line tend to keep moving unless compelled to stop"
This is his first law as stated in Newtons Principia in 1666; but 20 years earlier, during the English Civil War, Hobbes wrote in his Leviathan:
that when a thing lies still, unless somewhat else stir it, it will lie still forever, is a truth that no man doubts.
This was actually a truth first established by Aristotle; but only on Earth; in the heavens he assumed that the natural motion was not rest but circular motion. Hobbes however goes on to say:
But [the proposition] that when a thing is in motion it will eternally be in motion unless somewhat else stay it, though the reason be the same (namely that nothing can change itself), is not so easily assented to.
Its also known that Newton read Lucretious's epic cosmological poem De Rerum Natura on the Epicurean atomic theory of matter:
The opening exposition of book 2 descends into the details of atoms' behaviour and qualities. They are in perpetual motion at enormous speed, since in the void they get no resistance from the medium, and when they collide they can only be deflected, not halted.
Thus it is friction (collison in his words) that slows moving atoms about; its worth pointing out just how close he was to the 19C atomic theory of matter as developed by Boyle & Dalton:
Their weight gives them an inherent tendency to move downwards, but collisions can divert those motions in other directions. The result is that, when in a cosmic arrangement, atoms build up complex and relatively stable patterns of motion, which at the macroscopic level appear to us as states of rest or relatively gentle motion.
It was Gassendi round about the same time that Hobbes was writing who made atomism respectable again in early modern Europe:
The essential feature of atoms which does the most work in Gassendi's physics...is their inherent weight, which gives them an intrinsic, natural tendency to move.
And he developed the notion of constant motion:
Given this tendency, atomic rest is either provisionary or else an illusion. Atomic weight gives rise not only to a simple capacity for constant motion, but also to a range of more complex behaviors:
Its also worth noting that Aristotle had a theory of gravity - the natural motion of bodies; though of course he didn't call it by that name; its Newtons achievement to universalise that phenomena; he breached the division between the celestial & terrestial sphere - thus 'universal gravity'.