 Newton seems here to be arguing as follows: since rays of light have colors as basic features, we should regard these colors as qualities or properties of the rays (despite the fact that these properties are imperceptible under any ordinary circumstance); but doing so requires us to think of the rays as bearers of qualities, which is to say, as substances in their own right. And if rays of light are substances, this means that we cannot also think of them as qualities or properties of anything else. This last point follows from a widely accepted notion of a substance at the time, one easily found in  Descartes[7],  viz., that substances are those items that can exist independently of other items (whether they can exist independently even of God is a further question that we can ignore here). And if we cannot think of rays of light as properties or qualities, then they are not waves, for waves are features of some medium—think of waves on the surface of a lake. Newton concludes: light is a stream of particles (he does use the word ‘perhaps’ to hedge a bit here). Clearly, philosophical argumentation is a significant aspect of Newton’s reasoning in this paper, as are various philosophical concepts. It is intriguing to ponder the question, what overall conception of “sensible qualities” does Newton presuppose in this piece? If a ray of sunlight passes through my window, the fact that it appears white to me does not undermine Newton’s view (or so he thinks) that the ray actually contains a series of colors as its “qualities”. Are these qualities “sensible” if their presence can be detected only through the use of one or more prisms but never through the inspection of the sunlight through ordinary means (unaided perception, glasses, a magnifying glass, etc.)? These are apt to strike us as canonical philosophical problems.