In classical mathematics, an element of a poset is positive if and only if it not a bottom element.
A more subtle definition is needed in constructive mathematics; analogously to how “nonempty sets” may need to be replaced by inhabited sets. In mathematics that is both constructive and predicative, positivity can be axiomatised but not defined.
Let $L$ be a preordered set, and let $x$ be an element of $L$.
(in classical mathematics, and in predicative mathematics with classical logic)
An element $x \in L$ is positive if it is not a bottom element, equivalently if there exists an element $y \in L$ such that $x \leq y$ is false: $\neg(x \leq \bigvee \empty)$.
(in classical mathematics, and in impredicative constructive mathematics)
An element $x \in L$ is positive if whenever $x$ is bounded above by a join of some subset $A$ of $L$, $A$ is inhabited: $\forall A\,; x \leq \bigvee A \;\Rightarrow\; \exists\, u \in A$.
(in predicative constructive mathematics)
A positivity predicate on $L$ is a predicate $\lozenge x$, pronounced “$x$ is positive”, such that
If $x$ is bounded above by a join of a subset and $x$ is positive, then some element of that subset is positive: $\lozenge x \;\wedge\; x \leq \bigvee A \;\vdash\; \exists\, u \in A,\; \lozenge u$.
If $x$ is bounded above by a join on the assumption that $x$ is positive, then $x$ really is so bounded: $\lozenge x \;\Rightarrow\; x \leq \bigvee A \;\vdash\; x \leq \bigvee A$.
Any two of these definitions can be shown to be equivalent in the union of the corresponding foundational systems. Specifically:
In predicative mathematics with classical logic, one can prove that every preorder has a unique positivity predicate, which must match Definition 1.
In impredicative constructive mathematics, one can prove that if a preorder admits a positivity predicate, it is given by Definition 2. However, in general Definition 2 gives not rise to a positivity predicate in the sense of Definition 3.
In classical mathematics (with classical logic and impredicativity), all of the definitions are equivalent.
Every power set has a positivity predicate: $x$ is positive iff $x$ is inhabited. (Of course, there are few power sets in predicative mathematics, but often it is enough to think of the power set as a proper class.)
The positive predicate on a locale plays a role in the definition of an overt space. Locale theory is often considered constructively but impredicatively; in predicative constructive mathematics, a positivity predicate is used in the corresponding theory of formal topology.
In impredicative constructive mathematics, a sufficient condition for a complete poset to possess a positivity predicate is that any element is a join of positive elements. To show this, it suffices to prove that for any element $a$, it holds that $a = \bigvee \{ a | a \, positive \}$. The remaining properties can then be easily verified. By assumption, $a = \sup B$ for some set $B$ containing only positive elements. For any $b \in B$, it holds that $b \leq a$, thus that $a$ is positive and thus that $b \leq \bigvee \{ a | a \, positive \}$. Therefore $\sup B \leq \bigvee \{ a | a \, positive \}$ holds as well. The other inequality is trivial.
Every atom of $L$ is positive, and indeed an atom is precisely a minimal positive element.
Although classically trivial, a key property of positivity in the constructive context is this:
where $A^+$ is the set of positive elements of $A$.
A similar property is
for any element $a$.