  He says that this principle is “not found in Aldrich,” but that it is “in frequent  use.”[24]  This “frequent use” continued; later nineteenth and early twentieth century text books in England and America continued to endorse obversion (also called “infinitation” or “permutation”), and contraposition (also called “illative  conversion”).[25]  This full nineteenth century tradition is consistent only on the assumption that empty (and universal) terms are prohibited, but authors seem unaware of this; Keynes 1928, 126, says generously “This assumption appears to have been made implicitly in the traditional treatment of logic.” De Morgan is atypical in making the assumption explicit: in his 1847 text (p. 64) he forbids universal terms (empty terms disappear by implication because if A is empty, non-A will be universal), but later in the same text (p. 111) he justifies ignoring empty terms by treating this as an idealization, adopted because not all of his readers are  mathmeticians.[26]