Since to be body is just to be extended, Descartes believes that there is no such thing as empty space. What we typically think of as empty space between objects—for instance, the foot of air between the bed and the floor—is just insensible body. That space between the bed and the floor has extension. It is one foot by five feet by six feet. Therefore, it is body.

The only difference between space and the objects we think of as body, is that space has no sensible qualities. We cannot see space, or smell it, or feel it. Other than that, though, it is no different from a car or a bug or a planet.

Since the entire world is filled with body (there is not empty space between bodies) Descartes calls the entire universe a plenum, meaning that it is filled. The idea that space is a plenum leads to many interesting conclusions, such as the vortex theory of planetary motion, the globular theory of light, and the important proof that terrestrial and celestial matter are exactly the same.