Applying the Mathematical Continuum to Physical Space and Time: Following a lead given by Russell (1929, 182–198), a number of philosophers—most notably Grünbaum (1967)—took up the task of showing how modern mathematics could solve all of Zeno’s paradoxes; their work has thoroughly influenced our discussion of the arguments. What they realized was that a purely mathematical solution was not sufficient: the paradoxes not only question abstract mathematics, but also the nature of physical reality. So what they sought was an argument not only that Zeno posed no threat to the mathematics of infinity but also that that mathematics correctly describes objects, time and space. It would not answer Zeno’s paradoxes if the mathematical framework we invoked was not a good description of actual space, time, and motion! The idea that a mathematical law—say Newton’s law of universal gravity—may or may not correctly describe things is familiar, but some aspects of the mathematics of infinity—the nature of the continuum, definition of infinite sums and so on—seem so basic that it may be hard to see at first that they too apply contingently. But surely they do: nothing guarantees a priori  that space has the structure of the continuum, or even that parts of space add up according to Cauchy’s definition. (Salmon offers a nice example to help make the point: since alcohol dissolves in water, if you mix the two you end up with less than the sum of their volumes, showing that even ordinary addition is not applicable to every kind of system.) Our belief that the mathematical theory of infinity describes space and time is justified to the extent that the laws of physics assume that it does, and to the extent that those laws are themselves confirmed by experience. While it is true that almost all physical theories assume that space and time do indeed have the structure of the continuum, it is also the case that quantum theories of gravity likely imply that they do not. While no one really knows where this research will ultimately lead, it is quite possible that space and time will turn out, at the most fundamental level, to be quite unlike the mathematical continuum that we have assumed here.