In logic, the principle of double-negation shift (DNS), also known as the Kuroda Principle, is the statement that for any predicate , the following holds:
Of course, this is a tautology in classical logic where double negation is the identity, but in constructive mathematics this is not provable (although its converse? is).
It is provable even constructively, though, if the domain of quantification of the variable is finite. In general, the principle of double-negation shift can be considered a property of this domain, which holds automatically for finite sets, and might hold for some other sets but not for others. Sometimes by “double-negation shift” is meant specifically the version for the domain of natural numbers.
The principle of double-negation shift (over arbitrary domains) is equivalent to the double-negation of the universally-quantified principle of excluded middle:
Since is a constructive tautology, the above double-negated PEM follows from DNS with domain and . Conversely, if double-negated PEM holds, then to prove any negated goal like we may assume full PEM, i.e. classical logic, in which case DNS holds.
The interpretation of the predicate in an elementary topos is the subobject . Thus, holds in precisely if there exists a subterminal such that and
If is the topos of sheaves on a locale , the subterminals correspond to the opens of the locale, and holds for an open precisely if for all with . This means precisely that is boolean as an open sublocale, so double negation shift holds in precisely if has a dense boolean open sublocale. If is (the set of opens in) a -space, the booleanness condition is equivalent to discreteness, so double negation shift holds in the sheaf topos precisely if has a dense discrete open subspace. A non-trivial example satisfying is the 1-point compactification of an infinite set, and the real line is a counterexample (since a dense discrete open would be non-empty, and each of its element would be an isolated point, contradicting connectedness of ).
In the case of a presheaf topos, say on a small category , truth values are precisely sieves in , i.e. sets of objects such that for every morphism , implies . In this situation, is equivalent to for all . From this it can be deduced that double negation shift holds in presheaves over if and only if for every there exists an with .
DNS holds in every Kripke model with finite frame; see here.
Last revised on April 27, 2020 at 13:16:31. See the history of this page for a list of all contributions to it.