If k is a field, A a finite dimensional k-algebra, then the simple A-modules form a simple minded collection in the derived category D b (mod A). Their extension closure is mod A; in particular, it is abelian. This situation is emulated by a general simple minded collection S in a suitable triangulated category C . In particular, the extension closure S is abelian, and there is a tilting theory for such abelian subcategories of C . These statements follow from S being the heart of a bounded t-structure.It is a defining characteristic of simple minded collections that their negative self extensions vanish in every degree. Relaxing this to vanishing in degrees {−w +1, . . . , −1} only, where w is a positive integer, leads to the rich, parallel notion of w-simple minded systems, which have recently been the subject of vigorous interest.If S is a w-simple minded system for some w 2, then S is typically not the heart of a t-structure. Nevertheless, using different methods, we will prove that S is abelian and that there is a tilting theory for such abelian subcategories. Our theory is based on Quillen's notion of exact categories.