However, the path integral formulation is also extremely important in direct application to quantum field theory, in which the "paths" or histories being considered are not the motions of a single particle, but the possible time evolutions of a field over all space. The action is referred to technically as a functional of the field: S [ ϕ ], where the field ϕ (x) is itself a function of space and time, and the square brackets are a reminder that the action depends on all the field's values everywhere, not just some particular value. One such given function ϕ (x) of spacetime is called a field configuration. In principle, one integrates Feynman's amplitude over the class of all possible field configurations.