On the one hand, computation is a technique of abstraction. Layers of abstractions are piled up [...] abstraction is a self-contained dimension of existence of the computational. Historically and conceptually, computation draws upon the formal abstractions of logic and mathematics. Abstract mechanisms of inference drive it, while formal languages and symbolic manipulation are among the abstract means that ground the very possibility of algorithmic 'effective procedures'. On the other hand, however, computation is as concrete as the world in which it participates. Computation not only abstracts from the world in order to model and represent it; through such abstractions, it also partakes in the world. In this sense, computation is a technology of material agency [...]