  The possibility of extending the quantum mechanical superposition principle
to free parameters such as the cosmological constant appearing in the
Lagrangian of physical theories is examined. If the cosmological constant is
subject to a quantum mechanical superposition principle, its observed value is
a weighted average of its two natural values, one at the Planck scale and the
other at zero. As zero and nonzero values of the cosmological constant leads to
topologically distinct spacetimes, the amplitude for the cosmological constant
to take a Planck scale value is weighted over a topological Euclidean action
and the expectation value of the cosmological constant is found to be of the
order of its present observed value.
