 Riemann’s ideas in turn influenced Hermann von Helmholtz, who published several influential essays on how our knowledge of geometry is possible. In his “On the factual [thatsächlichen] foundations of geometry” (1866) he endeavoured to show how only a limited number of Riemannian geometries can be constructed in which there is a concept of rigid body motion. He argued that it is our experience of rigid bodies that teaches us what space is like, and in particular what distance is. He further claimed that a 2-dimensional space that admits rigid body motions would either be the Euclidean plane or the sphere. Beltrami wrote to him to point out that he had overlooked the possibility of non-Euclidean geometry, and Helmholtz not only agreed, but wrote a further essay (1868) in which he explained how it would be possible for us to have knowledge of this geometry at an intuitive level (thus giving the lie to followers of Kant). Many Kantians refused to be convinced, but one person that these ideas (and Riemann's) very likely influenced was Henri Poincaré (see Gray 2012).