Despite their invaluable contribution to the programming language community,
monads as a foundation for the study of effects
have three problems: they make it difficult to combine effects;
they enforce sequentialization of computations by the syntax;
they prohibit effect-free evaluations from invoking effectful computations.
Building on the judgmental formulation and the possible worlds interpretation
of modal logic, 
we propose a logical analysis of effects based upon the view  
monads are not identified with effects.
Our analysis leads to a language called $\langmonad$ 
which distinguishes between control effects and world effects,
enforces sequentialization of computations only by the semantics,
and logically explains the invocation of computations from evaluations.
$\langmonad$ also serves as a unified framework for studying 
Haskell and ML, which have traditionally been studied separately.