# Contents

## Definition

A Boolean topos is a topos that is also a Boolean category.

There are several conditions on a topos that are necessary and sufficient to be boolean:

• Every subobject has a complement (the general definition of boolean category).
• Every subobject lattice is a Boolean algebra.
• The subobject classifier $\Omega$ is an internal Boolean algebra.
• The maps $\top ,\perp :1\to \Omega$ are a coproduct cone (so in particular, $\Omega \cong 1+1$, but this alone is not sufficient).

## As a context for foundations

The internal logic of a boolean topos with natural numbers object can serve as foundations for “ordinary” mathematics, except for that which relies on the axiom of choice. If you add the axiom of choice, then you get (an internal version of) ETCS; conversely, if you use an arbitrary topos, then you get constructive mathematics. (For some high-powered work, you may also need to add a version of the axiom of replacement or an axiom of Grothendieck universes.)

Every cartesian closed boolean pretopos is in fact a topos. This is why ‘generalised predicativism’ (with function types but not power types) is necessarily a feature of constructive mathematics only.

## Examples

With excluded middle in the meta-logic, every well-pointed topos is a Boolean topos. This includes Set and models of ETCS.

The topos of canonical sheaves on a Boolean algebra is Boolean.

If $E$ is any topos, the category of sheaves for the double-negation topology is a Boolean subtopos of $E$.

Any topos satisfying the axiom of choice is Boolean. This result is due to R. Diaconescu; see excluded middle for a brief discussion.

Barr's theorem implies that any topos $E$ can be covered by a Boolean topos $F$, in the sense of there being a surjective geometric morphism $f:F\to E$.

Revised on August 19, 2012 22:07:03 by Todd Trimble (67.81.93.25)