  We study the principle phi implies box phi, known as `Strength' or `the
Completeness Principle', over the constructive version of L\"ob's Logic. We
consider this principle both for the modal language with the necessity operator
and for the modal language with the Lewis arrow, where L\"ob's Logic is
suitably adapted.
  Central insights of provability logic, like the de Jongh-Sambin Theorem and
the de Jongh-Sambin-Bernardi Theorem, take a simple form in the presence of
Strength. We present these simple versions. We discuss the semantics of two
salient systems and prove uniform interpolation for both. In addition, we
sketch arithmetical interpretations of our systems. Finally, we describe the
various connections of our subject with Computer Science.
