nLab
false

Contents

Idea

In logic, the false proposition, called falsehood or falsity, is the proposition which is always false.

The faleshood is commonly denoted falsefalse, FF, \bot, or 00.

Definitions

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, falsefalse 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 a topos

In terms of the internal logic of a topos (or other category), falsefalse 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 falsefalse and truetrue.

Examples

In the topos SetSet

In the archetypical topos Set, the terminal object is the singleton set ** (the point) and the poset of subobjects of that is classically {*}\{\emptyset \hookrightarrow *\}. Then falsehood is the empty set \emptyset, 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 falsefalse in Set or ∞Grpd may canonically be thought of as being the unique empty groupoid.

Revised on August 11, 2013 17:21:30 by Urs Schreiber (89.204.135.38)