 (Some of) the rules of conversion for categorical propositions constitute another major departure from Aristotle. On Avicenna’s model, the absolute universal negative proposition, X1-e, fails to convert due to its being a one-sided temporal proposition (in general it is not true that if no A is always B, then no B is always A, as in “No animal always sleeps” and “Nothing that sleeps is always an animal”). L-a and L-i propositions do not convert as L-i propositions but rather as M1-i propositions (for a general presentation and an account of the proofs see Street 2002, Thom 2003). L-e and Ld1-e propositions convert as such. In the first case, the proof is one of the most controversial points in Avicenna’s system, directly connected with the validity of first-figure syllogisms with a one-sided possibility, M1, minor premise. The conversion of Ld1-propositions as Ld1-propositions is in fact the minimal essential requirement for the productivity of second-figure moods involving absolutes in XX and XM mixes. For Avicenna’s account of why an Ld1-proposition may legitimately be regarded as an absolute proposition, see Qiyās I.4 and Burhān II.1, discussed in (Strobino forthcoming).