  The present section states some elements of the historical and systematic background of the KS theorem. Most importantly, an argument by von Neumann (1932), a theorem by Gleason (1957), and a critical discussion of both plus a later argument by Bell (1966) have to be considered. Von Neumann, in his famous 1932 book Die mathematischen Grundlagen der Quantenmechanik, disputed the possibility of providing QM with an HV underpinning. He gave an argument that boils down to the following: Consider the mathematical fact that, if A and B are self-adjoint operators, then any real linear combination of them (any C =   αA +  βB, where  α,  β are arbitrary real numbers) is also a self-adjoint operator. QM further dictates that: