Now, if I go farther, and for weighty reasons rank as mere appearances the remaining qualities of bodies also, which are called primary, such as extension, place, and in general space, with all that which belongs to it (impenetrability or materiality, space, etc. )—no one in the least can adduce the reason of its being inadmissible." This follows directly from Kant's transcendental idealism, according to which space and time are mere forms of intuition, which means that any quality that can be attributed to the spatiotemporal objects of experience must be a quality of how things appear to us rather than of how things are in themselves.
Kant's transcendental idealism, which posits that space and time are merely forms of intuition, meaning any quality attributed to spatiotemporal objects is a quality of how they appear to us rather than their true nature.