 Finally, it is worth mentioning that the status of the field in algebraic QFT differs from that in Wightman’s axiomatic QFT. In both approaches, a field is an abstract system having an infinite number of degrees of freedom. Sub-atomic quantum particles are field effects that appear in special circumstances. In algebraic QFT, there is a further abstraction: the most fundamental entities are the elements of the algebra of local (and quasi-local) observables, and the field is a derived notion. The term local means bounded within a finite spacetime region, and an observable is not regarded as a property belonging to an entity other than the spacetime region itself. The term quasi-local is used to indicate that we take the union of all bounded spacetime regions. In short, the algebraic approach focuses on local (or quasi-local) observables and treats the notion of a field as a derivative notion; whereas the axiomatic approach (as characterized just above) regards the field concept as the fundamental notion. Indeed, it is common practice for proponents of the algebraic approach to distance themselves from the field notion by referring to their theory as “local quantum physics”. The two approaches are mutually complementary – they have developed in parallel and have influenced each other by analogy (Wightman 1976). For a discussion of the close connections between these two approaches, see Haag (1996, p. 106).