| Obversion is discussed in several twelfth and early thirteenth century anonymous texts edited in De Rijk 1967, where it is seen as a type of equipollence. |
| Similar results follow for contraposition and obversion. |
| Aristotle discussed some instances of obversion in De Interpretatione. |
| When a proposition is posed against the proposition that results from changing its quality at the same time that its second term is negated, the resulting equivalence is called obversion. |
| In this it differs from Obversion, Conversion, and contraposition, each of which stands for one process. |
| Because the obverse of any categorical proposition is logically equivalent to it, obversion is a form of immediate inference. |
