the opposite direction by exception.
Now that which is must needs be when it is, and that which is not
must needs not be when it is not. Yet it cannot be said without
qualification that all existence and non-existence is the outcome of
necessity. For there is a difference between saying that that which
is, when it is, must needs be, and simply saying that all that is must
needs be, and similarly in the case of that which is not. In the case,
also, of two contradictory propositions this holds good. Everything
must either be or not be, whether in the present or in the future, but
it is not always possible to distinguish and state determinately which
of these alternatives must necessarily come about.
Let me illustrate. A sea-fight must either take place to-morrow or
not, but it is not necessary that it should take place to-morrow,
neither is it necessary that it should not take place, yet it is
necessary that it either should or should not take place to-morrow.
Since propositions correspond with facts, it is evident that when in
future events there is a real alternative, and a potentiality in
contrary directions, the corresponding affirmation and denial have the
This is the case with regard to that which is not always existent or
not always nonexistent. One of the two propositions in such
instances must be true and the other false, but we cannot say
determinately that this or that is false, but must leave the
alternative undecided. One may indeed be more likely to be true than
the other, but it cannot be either actually true or actually false. It
is therefore plain that it is not necessary that of an affirmation and
a denial one should be true and the other false. For in the case of
that which exists potentially, but not actually, the rule which
applies to that which exists actually does not hold good. The case
is rather as we have indicated.