Before mathematicians assert something (other than an axiom) they are supposed to have proved it true. What, then, do mathematicians mean when they assert a disjunction
P Q, where
P and
Q are syntactically correct statements in some (formal or informal) language that a mathematician can use? A natural — although, as we shall see, not the unique — interpretation of this disjunction is that not only does (at least) one of the statements
P,
Q hold, but also we can decide which one holds. Thus just as mathematicians will assert that
P only when they have decided that
P by proving it, they may assert
P Q only when they either can decide — that is, prove — that
P or decide (prove) that
Q.
With this interpretation, however, mathematicians run into a serious problem in the special case where
Q is the negation, ¬
P, of
P. To decide that ¬
P is to show that
P implies a contradiction (such as 0=1). But it will often be that mathematicians have neither decided that
P nor decided that ¬
P. To see this, we need only reflect on the following:
Goldbach Conjecture:
Every even integer > 2 can be written as a sum of two primes,
which remains neither proved nor disproved despite the best efforts of many of the leading mathematicians since it was first raised in a letter from Goldbach to Euler in 1742. We are forced to conclude that, under the very natural interpretation of
P Q just canvassed, only an optimist can retain a belief in the law of excluded middle,
P ¬
P.
Traditional, or
classical, mathematics gets round this by widening the interpretation of disjunction: it interprets
P Q as ¬(¬
P¬
Q), or in other words, “it is contradictory that both
P and
Q be false”. In turn, this leads to the
idealistic interpretation of existence, in which
xP(
x) means ¬
x¬
P(
x) (“it is contradictory that
P(
x) be false for every
x”). It is on these interpretations of disjunction and existence that mathematicians have built the grand, and apparently impregnable, edifice of classical mathematics which serves a foundation for the physical, the social, and (increasingly) the biological sciences. However, the wider interpretations come at a cost: for example, when we pass from our initial, natural interpretation of
P Q to the unrestricted use of the idealistic one, ¬(¬
P¬
Q), the resulting mathematics cannot generally be interpreted within computational models such as recursive function theory
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