The extension of the senses and their bodily and extra-bodily localization and simultaneity seem to be based on spatiotemporal structures. Although the individual senses differ from each other in their nature as secondary qualia, they can be analogous to each other with regard to their common spatiotemporal basis. When Princess Elisabeth of Bohemia in her first letter to Descartes (dt. 6 May 1643) polemicizes against the exclusively mental reduction of the bodily volition and sensory perceptions in Descartes’ Meditations, it also points in a broader sense to the impossibility of a transcendental reduction of space and time as pure forms of a priori sensibility, as was undertaken later by Kant. Descartes’ confession to the princess that he strategically ignored the bodily acts of volition and sensory perceptions in favour of the mere mental act of thinking also marks the triumph of these fundamental primary qualities—which are autonomous from the thinking and sensibly perceiving subject—over a Cartesian-rational and Kantian-transcendental reduction. Such an objective autonomy gives space and time, or rather the spatiotemporal structures, the existence and permanence of a transcendental-subjectively irreducible skeleton of reality onto which sensory qualities extend like flesh. This points to the necessary participation of the material reality of the body and extra-bodily phenomena in the sensory perceptions, which, on the one hand, requires a synthetic-compositional epistemology between philosophy and natural science and, on the other hand, justifies the fundamental analogicity between the bodily and the extra-bodily extension of sensory perceptions.