In logic, the false proposition, called falsehood or falsity, is the proposition which is always false.
The faleshood is commonly denoted , , , or . These may be pronounced ‘false’ even where it would be ungrammatical for an adjective to appear in ordinary English.
In classical logic
In classical logic, there are two truth values: false and true. Classical logic is perfectly symmetric between falsehood and truth; see de Morgan duality.
In constructive logic
In constructive logic, is the bottom element in the poset of truth values.
Constructive logic is still two-valued in the sense that any truth value is false if it is not true.
In linear logic
In linear logic, there is both additive truth, denoted , and multiplicative truth, denoted . Despite the notation, it is that is the bottom element of the lattice of linear truth values. (In particular, but .)
In a topos
In terms of the internal logic of a topos (or other category), is the bottom element in the poset of subobjects of any given object (where each object corresponds to a context in the internal language).
However, not every topos is two-valued, so there may be other truth values besides and .
In type theory
In type theory with propositions as types, falsehood is represented by the empty type.
In homotopy type theory
In homotopy type theory, falsehood is represented by the empty space.
In the topos
In the archetypical topos Set, the terminal object is the singleton set (the point) and the poset of subobjects of that is classically . Then falsehood is the empty set , seen as the empty subset of the point. (See Internal logic of Set for more details).
The same is true in the archetypical (∞,1)-topos ∞Grpd. From that perspective it makes good sense to think of
In this sense, the object in Set or ∞Grpd may canonically be thought of as being the unique empty groupoid.