  We prove that the forgetful functor from the category of Boolean inverse
semigroups to inverse semigroups with zero has a left adjoint. This left
adjoint is what we term the `Booleanization'. We establish the exact connection
between the Booleanization of an inverse semigroup and Paterson's universal
groupoid of the inverse semigroup and we explicitly compute the Booleanization
of the polycyclic inverse monoid $P_{n}$ and demonstrate its affiliation with
the Cuntz-Toeplitz algebra.
