  Doctrines are categorical structures very apt to study logics of different
nature within a unified environment: the 2-category Dtn of doctrines. Modal
interior operators are characterised as particular adjoints in the 2-category
Dtn. We show that they can be constructed from comonads in Dtn as well as from
adjunctions in it, and the two constructions compare. Finally we show the
amount of information lost in the passage from a comonad, or from an
adjunction, to the modal interior operator.
  The basis for the present work is provided by some seminal work of John
Power.
