There will always be sludge that frustrates our efforts to implement the ideal in practice, not only in the present case but on all occasions. Sludge!… that unruly, refractory element in the world that, assuming the form of mechanical friction in one case, renders the perpetual motion machine impossible and that, assuming the form of Gödel’s theorem in another, renders the consistency of mathematics incapable of proof. Sludge is everywhere, it cannot be escaped, not only in the physical world but even in the realm of pure ideas. Although this principle of recalcitrance is familiar to all, it has been generally believed to be merely contingent in nature, a brute fact that has no intelligible warrant. — Jose Benardete (1964)