Foucault puts forward four hypotheses to "describe statements in the field discourse and the relations of which they are capable (p. 34).
First, "statements, different in form and dispersed in time, form a group if they refer to one and the same object" (p.35).
Second, statements characterised by "a certain style, a certain constant manner of statement" (p.36) form a discourse.
Third, groups of statements showing a system of permanent and coherent concepts involved" (p.38) form a discourse.
Fourth, discourses can be recognised by the presence of persistent themes.
Foucault defines discourse in the following way: "the group of statements that belong to a single system of formation" (1972, p.121).