Handbook of the History of Logic: - Fordham University Faculty

164 Terence Parsons
true. This may not accord well with ordinary speakers of Latin, but logicians
insisted that this was the right way to read the proposition. This is part of the
regimentation that was mentioned earlier. It may be defended in the way that any
regimentation is defended, by claiming that it is useful for logical purposes, even
if it does not conform well to ordinary usage.
Of course, this proposal will not work unless other parts of logic are formulated
with this in mind. For example, we do not want to include the validity of ‘Some
AisnotB∴ Some A is’. This inference, of course, is not considered valid.
I said that these proposals make for theoretical simplicity, but I didn’t say how.
One simplicity is that it makes it possible to maintain these principles:
Affirmative categorical propositions are false when any of their main
terms are empty.
Negative categorical propositions are true when any of their main terms
are empty.
These principles hold for singular propositions and for the forms of categorical
propositions we have discussed; they will hold without exception, even when categorical
propositions are expanded far beyond the forms that Aristotle discussed.
1.5 Conversions
The square of opposition deals with logical relations among propositions which
have the same subject and same predicate. There are also other relations. If a
proposition is generated from another by interchanging the original subject and
predicate, those propositions are candidates for the logical relation of “conversion”:
7
Simple conversion: A proposition is said to convert simply if it entails the result
of interchanging its subject and predicate terms. Universal Negative and Particular
Affirmative propositions convert simply, resulting in equivalent propositions:
‘NoAisB’ converts simply to ‘No B is A’
‘Some A is B’ converts simply to ‘Some B is A’.
Conversion per Accidens (Accidental conversion): Whereas simple conversion
produces a proposition equivalent to the original, conversion per accidens is not
symmetric. A universal proposition may be converted per accidens: you interchange
its subject and predicate terms and change the universal sign to a particular
sign (adding a negation, if necessary, to preserve quality). This inference is
not reversible.
7 The conversion laws stated below were proved by Aristotle in Prior Analytics I.2. They
appear (usually without proof) in every major logic text.