  We extend Bourke and Garner's idempotent adjunction between monads and
pretheories to the framework of $\infty$-categories and we use this to prove
many classical results about monads in the $\infty$-categorical framework.
Amongst other things, we show that the category of algebras for an accessible
monads on a locally presentable $\infty$-category $\mathcal{E}$ is again
locally presentable, and that a diagram of accessible monads on a locally
presentable $\infty$-category admits a colimit. Our results also provide a new
and simpler way to construct and describe monads in terms of theories.
