Foucault’s method was to ask, it has been said,

what rules permit certain statements to be made; what rules order these statements; what rules permit us to identify some statements as true and others as false; what rules allow the construction of a map, model or classificatory system, what rules are revealed when an object of discourse is modified or transformed … Whenever sets of rules of these kinds can be identified, we are dealing with a discursive formation or discourse.

Source: J. B. Harley, Deconstructing the map Reprinted from Cartographica, v. 26, n. 2 (Spring 1989), 1-20.