 As a concrete example of a judgment of relative spatial position, Helmholtz gives the case of someone grasping a pen in her fingers. She cannot infer directly from the sensation of the pen that it is in one place, because each finger feels only the position of the pen relative to the finger itself. She would have exactly the same sensations if her fingers were touching two or three different pens, separated in space. The belief that the pen is in one place only is based on her knowledge that his fingers are close enough together that only one pen will fit between them. As Helmholtz remarks,