t the intervention of another premise or middle term. A
proposition is said to be "converted" when the subject and the predicate
change places; the original proposition is the "convertend," the new one
the "converse." The chief rule governing conversion is that no term
which was not _distributed_[1] in the convertend may be distributed in
the converse; nor may the quality of the proposition (affirmative or
negative) be changed. It follows that of the four possible forms of
propositions A, E, I and O (see article A), E and I can be converted
simply. If no A is B (E), it follows that no B is A; if some A is B, it
follows that some B is A. This form of conversion is called Simple
Conversion; E propositions convert into E, and I into I. On the other
hand, A cannot be converted simply. If all men are mortal, the most that
can follow by conversion is that some mortals are men. This is called
Conversion by Limitation or _Per Accidens_. Only if it be known from
external or non-logical sources that the predicate also is distributed
can there be simple conversion of a universal affirmative. Neither of
these forms of conversion can be applied to the particular negative
proposition O, which has to be dealt with under a secondary system of
conversion, as follows. The terminology by which these secondary
processes are described is not altogether satisfactory, and logicians
are not agreed as to the application of the terms. The following system
is perhaps the most commonly used. We have seen that the converse of
"all A is B" is "some B is A"; we can, in addition, derive from it
another, though purely formal, proposition "no A is not-B"; i.e. an E
proposition. This process is called Obversion, Permutation or Immediate
Inference by Privative Conception; it is applicable to every proposition
including O. A further process, known as Contraposition or Conversion by
Negation, consists of conversion following on obversion. Thus from "all
A is B," we get "no not-B is A." In the case of the O proposition we get
(by obversion) "some A is not-B" and then (by conversion) "some not-B is
A" (i.e. an I proposition). In the case of the I proposition the
contrapositive is impossible, as infringing the main rule of conversion.
Another term, Inversion, has been used by some logicians for a still
more complicated process by the alternative use of conversion and
obversion, which is applicable to A and E, and results in obtaining a
proposition concerning the