The interior of Borromini's small church consists of a multitude of blended geometries, of which the remaining partial volumes are indicating their missing parts. In observing the space, our perceptional system will involuntarily add these missing parts to complete the indicated space to virtual volumes.

The niches of the altar and the entrance vis-à-vis appear as half cylinders. Protracting their contours will result in full cylinders that extend into the central space of the church. In a similar way, the halves of the oval cylinders that frame the side altars complete themselves in the space. In these side altar conches, arcade shaped niches establish a relationship to each other across the central space. The swinging, circumferential architrave emphasizes the contours of the basic geometries by offsetting and repeating them to the inside and upwards several times. Above, the calottes indicate those globes and ovaloids of which they are blended remains and finally, the cupola and lantern repeat the theme of the ovaloid and oval cylinder as the upper conclusion of the space.
The becoming and disappearance of these virtual volumes depend on the location and viewing direction of the observer. The spaces are ephemeral, transient. San Carlo therefore can be read as a multitude of indicated spaces of which every observer can construct his own dynamic and transient spatial experience.