Kœstler (1979: 535-6, 539):
The division of the world into 'primary' and 'secondary' qualities was completed by Descartes. He further reduced primary qualities to 'extension' and 'motion', which form the 'realm of extension' — res extensa — and he lumped together everything else in the res cogitans, the realm of the mind …
Descartes carried the process one step further [than Galileo] by paring down the reality of the external world to particles whose only quality was extension in space and motion in space and time.
From the perspective of Systemic Functional Linguistic theory, all such qualities, primary and secondary, extension and motion, exist "only in the observer's consciousness" — since they are all meanings construed of experience.