| A further process, known as contraposition or Conversion by Negation, consists of conversion following on obversion. |
| Because the obverse of any categorical proposition is logically equivalent to it, obversion is a form of immediate inference. |
| This identification of cause with ground leaves Vātsyāyana unclear about the difference between obversion and contraposition. |
| Aristotle discussed some instances of obversion in De Interpretatione. |
| 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. |
