Gilles Deleuze, 1977:

"Possibly we're in the process of experiencing a new relationship between theory and practice. At one time, practice was considered an application of theory, a consequence; at other times, it had an opposite sense and it was thought to inspire theory, to be indispensable for the creation of future theoretical forms. In any event, their relationship was understood in terms of a process of totalisation. For us, however, the question is seen in a different light. The relationships between theory and practice are far more partial and fragmentary. on one side, a theory is always local and related to a limited field, and it is applied in another sphere, more or less distant from it.
The relationship which holds in the application of a theory is never one of resemblance. Moreover, from the moment a theory moves into its proper domain, it begins to encounter obstacles, walls, and blockages which require its relay by another type of discourse (it is through this other discourse that it eventually passes to a different domain). Practice is a set of relays from one theoretical point to another, and theory is a relay from one practice to another. No theory can develop without eventually encountering a wall, and practice is necessary for piercing this wall."

-Gilles Deleuze (1977:165), with  Michel Foucault, in Language, Counter-Memory, Practice.  Cornell University Press 1977.