↑ Crews (1977, p. 13) ↑ Crews (1977, p. 13) ↑ Crews (1977, p. 16) ↑ Morrell (2006), p. 127 ↑ Selgin (2007), p. 38 ↑ Polking (1990), p. 106 ↑ Baldick (2004) ↑ Ludlow, Peter (2007), Descriptions, Stanford Encyclopedia of Philosophy