Aufbau §176 "demonstrating" the non-constructability of the real (as a mind-transcendent) concept had §17 of Weyl's 1926 book, Philosophie der Mathematik und Naturwissenschaften squarely in its sights. Weyl had argued that postulation of a real, external world is both necessary for natural science and that such an objective world can be constructed, but only in abstract mathematical symbols far removed ("distilled") from immediately given content. This objective world is a "symbolic construction of exactly the same kind as that which Hilbert carries through in mathematics". For Hilbert and Weyl, symbolic construction is the twentieth century manifestation of Kant's regulative idea of unity of nature.