The monad axioms can be seen at work in a simple example: let G be the forgetful functor from the category **Group** of groups to the category **Set** of sets. Then as F we can take the free group functor. This means that the monad T = FoG takes a group X and returns the free group Free(X) on the set underlying X. What we are given here consists of two observations: X ->T(X) by including any group X in Free(X) in the natural way. Further, T(T(X)) -> T(X) can be made out of a natural concatenation of 'strings of strings'. This amounts to two natural transformations I -> T, and ToT -> T. They will satisfy some axioms about identity and associativity based on the monoid axioms.

Hence in fact the name **monad**. Those axioms are taken as the definition of a general monad (not assumed *a prior* to be connected to an adjunction) on a category. Two constructions, the *Kleisli category* and the category of *Eilenberg-Mac Lane algebras*, are extremal solutions of the problem of constructing an adjunction starting with a given monad.

While monads are quite common, making them explicit is less so (the language belongs to the school of Mac Lane, and has rarely been used in the school of Grothendieck, which prefers to write out monads and comonads longhand). In categorical logic, an analogy has been drawn between the monad-comonad theory, and modal logic.