 While Natorp often seems to blur the distinction between Kantian categories and ideas, collapsing them both into “hypotheses” (see below), there is one notion that does function as an ideal: the Ding an sich. According to Natorp, the thing in itself, properly conceived, is a “pure limit-concept [Grenzbegriff], which merely delimits [begrenzt] experience in its own creative legislation” (Natorp 1912c: 199); more precisely, the thing in itself is not some mind-independent X, but rather an X necessarily implied by the fact of scientific progress, namely the “comprehensive sum total [Inbegriff] of scientific knowledge” (Natorp 1918a: 19). Because science is an activity of reason alone, and the thing in itself is a rational posit necessary for that activity, it is no longer in conflict with the postulate of reason’s autonomy. Given an object of scientific cognition, the cognition is conceived as a process never “definitively concluded”, as Cohen says; rather, “every true concept is a new question; none is a final answer” (Cohen 1885, cited at Natorp 1918a: 19). Natorp comments: “Just this is the meaning of the thing in itself as X: the infinite task” (Natorp 1918a: 19). In other words, the thing in itself is the ideal of an object exhaustively determined by concepts, that is, completely known. As with Kant, however, our cognitive finitude means that the process of conceptual determination can only approach this ideal  asymptotically.[21]  This pursuit of total determination is what Natorp calls “method”, the pursuit of science.