  The situation changed radically with Grothendieck’s (1957) landmark paper entitled “Sur quelques points d’algèbre homologique”, in which the author employed categories intrinsically to define and construct more general theories which he (Grothendieck 1957) then applied to specific fields, e.g., to algebraic geometry. Kan (1958) showed that adjoint functors subsume the important concepts of limits and colimits and could capture fundamental concepts in other areas (in his case, homotopy theory).