 As noted by Lowe (1997) and Wright and Hale (2001) any two-level criterion can be restated in a one-level form (though, of course, not conversely). For example, to say that the direction of line a is identical with the direction of line b if and only if line a is parallel to line b is to say that directions are the same if and only if the lines they are of are parallel, which is the form of a one-level criterion. A way of unifying the various different ways of talking of criteria of identity is thus to take as the paradigmatic form of a statement of a criterion of identity a statement of the form: for any x, for any y, if x is an F and y is an F then x=y if and only if Rxy (Lowe 1989, 1997).