Spatial items which relate to each other as mirror-images do, so that although the same shape, they cannot be superimposed so as to occupy exactly the same volume of space. In three dimensions, a right hand cannot occupy exactly the same space as a left hand. On a two-dimensional plane, a letter L cannot be moved to cover the same letter L reversed. It is notable that in three-dimensional space the letter can be flipped over to cover the reversed version. Similarly, if there were a fourth dimension of space, we could in principle disappear from the three familiar dimensions, flip round, and return with, for instance, our hearts on the right-hand side of our bodies. Kant (Metaphysical Foundations of Natural Science, i. 13) advances incongruent counterparts as a problem for a purely relational theory of space. If we imagine a universe with just one object, say a single hand, it would seem to be determinate whether it is a left hand or a right hand, yet all the spatial relations of its elements will be the same whichever one it is. So a relational theory of space seems unable to account for the difference, and therefore seems to be inadequate.