Relativity tells us our universe has 4 dimensions, string theory says its 10, or 11 if you take a variant known as M-theory. What does the term “dimension” mean in physics and mathematics, and how can we know how many there are? In Aeon magazine, IFF director Margaret Wertheim has an essay looking at the history of this foundational concept. Wertheim writes: “The birth of modern science is usually seen as a transition into a mechanist account of nature, but arguably more important – and certainly more enduring – is the transition it entailed in our conception of space as a geometric construct.” From Descartes to Einstein, from relativity to string theory, and beyond, dimensionality is a crucial tool for understanding our world.