 They proceed by sketching an iconic thought experiment whose variations continue to be important and widely discussed. The experiment concerns two quantum systems that are spatially distant from one another, perhaps quite far apart, but such that the total wave function for the pair links both the positions of the systems as well as their linear momenta. In the EPR example the total linear momentum is zero along the x-axis. Thus if the linear momentum of one of the systems (we can call it Albert’s) along the x-axis were found to be p, the x-momentum of the other system (call it Niels’) would be found to be −p. At the same time their positions along x are also strictly correlated so that determining the position of one system on the x-axis allows us to infer the position of the other system along x. The paper constructs an explicit wave function for the combined (Albert+Niels) system that embodies these links even when the systems are widely separated in space. Although commentators later raised questions about the legitimacy of this wave function, it does appear to guarantee the required correlations for spatially separated systems, at least for a moment (Jammer 1974, pp. 225–38; see also Halvorson 2000). In any case, one can model the same conceptual situation in other cases that are clearly well defined quantum mechanically (see  Section 3.1).