Thus, in the case of those predications which have within them no
contradiction when the nouns are expanded into definitions, and
wherein the predicates belong to the subject in their own proper sense
and not in any indirect way, the individual may be the subject of
the simple propositions as well as of the composite. But in the case
of that which is not, it is not true to say that because it is the
object of opinion, it is; for the opinion held about it is that it
is not, not that it is.
As these distinctions have been made, we must consider the mutual
relation of those affirmations and denials which assert or deny
possibility or contingency, impossibility or necessity: for the
subject is not without difficulty.
We admit that of composite expressions those are contradictory
each to each which have the verb 'to be' its positive and negative
form respectively. Thus the contradictory of the proposition 'man
is' is 'man is not', not 'not-man is', and the contradictory of 'man
is white' is 'man is not white', not 'man is not-white'. For
otherwise, since either the positive or the negative proposition is
true of any subject, it will turn out true to say that a piece of wood
is a man that is not white.
Now if this is the case, in those propositions which do not
contain the verb 'to be' the verb which takes its place will
exercise the same function. Thus the contradictory of 'man walks' is
'man does not walk', not 'not-man walks'; for to say 'man walks'
merely equivalent to saying 'man is walking'.
If then this rule is universal, the contradictory of 'it may be'
is may not be', not 'it cannot be'.
Now it appears that the same thing both may and may not be; for
instance, everything that may be cut or may walk may also escape