Subtle matter is for Descartes, as for some ancient and medieval predecessors, the imperceptibly small parts of matter. Indispensable to any theorist denying the vacuum, subtle matter exists between observable bodies and within their pores (AT I 139–40, CSMK 21–22). Only one sort of subtle matter appears in the Dioptrics and Meteors, a highly fluid matter that fills the heavens and is responsible for light transmission. Although at this time Descartes describes subtle matter as “continuous” as well as “fluid” (AT I 417, CSMK 63), he later explains its fluidity in terms of the rapid motion of small constituent particles. In correspondence with Mersenne (AT II 483, CSM III 132–33), Descartes reveals that the term “subtle matter” actually encompasses two kinds of matter, and his Principles of Philosophy (1644) accordingly asserts, hypothetically, a total of three “elements” or kinds of matter (AT VIIIA 105, CSM I 258). In conformity to his identification of matter with extension, these differ only in size, shape, speed, and function (AT II 484–85, CSMK 133) and are thus transformable into one another through erosion and accretion (AT VIIIA 105, 142–44, CSM I 258, 262; AT VIIIA 206, CSM I 267). Subtle matter comprises primary and secondary matter, while the tertiary element constitutes the large particles of observable bodies, including the earth, other planets, and comets. Primary matter, which constitutes the sun and fixed stars (AT VIIIA 105, CSM I 258), includes the smallest and fastest-moving particles. Because these particles also fill the spaces (so to speak) between larger particles (AT VIIIA 104, 119, 144–45, 207, 213; CSM I 258, 261, 262, 268, 269), most lack any determinate shape, continually adjusting to those so-called spaces (angular-shaped exceptions are discussed at AT VIII 144, CSM I 262). Particles of secondary matter, a constituent of celestial vortices (AT VIIIA 105, CSM I 258), typically have determinate, spherical shapes, any corners having been worn away by collisions. These particles fill the pores of large bodies (primary particles filling the gaps between secondary ones) (AT VIIIA 103–4, CSM I 258).

The term “inertia” was first introduced by Kepler (Epitome of Copernican Astronomy, IV.3, 59; IV.2, 54, 55) to designate the opposition to motion that he considered natural to matter. One component of this opposition was a variant of Aristotelian doctrine about “violent motion” that Galileo, Gassendi, Beeckman, and Descartes would reject: persistence of motion requires some external agent, because bodies in motion tend toward rest. Although the Keplerian concept of inertia is sometimes erroneously identified with this first component alone, Kepler's view included a second component claim, a variant of which future physics would retain: bodies resist being put into motion, and their resistance varies with density (Kepler 1858 and 1896, 1:161; 6:174–75, 342; Jammer 1961, 57, 56, 55).

Descartes uses the term “inertia” much as Kepler did. In response to Debeaune's claim that resistance is due to a tardiveté naturelle, Descartes denies that bodies possess any such natural inertia or sluggishness. His own view emerging, he too acknowledges resistance, though correlating it with size rather than density, as his definition of matter in terms of extension alone demands. Diverging from Kepler, he pairs his recognition of resistance with the anti-Aristotelian claim that bodies in motion tend to stay in motion (AT II 466–67).

The term “inertia” eventually labeled a principle based upon Newton's first law of motion, which states, “Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by forces impressed” (Principia, 417). The concept of an inertial state is inseparable from such concepts as space and motion, and whereas Newton saw rest and uniform motion as distinct if humanly indistinguishable states, they were eventually classified as distinct only in connection with a chosen reference frame.