  The conception of science that Kant presents in the Preface has been the focus of considerable attention over the past several decades. In the German literature, the issues raised above have been discussed at length by Plaass (1965), Schäfer (1966), Hoppe (1969), Gloy (1976), and Cramer (1985). Pollok (2001) has recently produced a detailed and comprehensive textual commentary on the Metaphysical Foundations. Important work has also been done in the English literature by Walker (1974), Brittan (1978), Buchdahl (1968, 1969, and 1986), Parsons (1984), Butts (1986), and Watkins (1998a). Friedman (1992, 2001, 2002 and 2013) has been especially influential on these issues as well.