I ask a physical interpretation of the relationship between states of a quantum system introduced, in analogy with the causal relationship between events in Minkowski space-time by means of the formula xRy if and only if < x-y/ T( x-y)> > or = 0. Here x, y are vectors of a Hilbert space and T is a Hermitian operator applied in it. In the original relation x,y are Minkowski space-time four- vectors and T is the metric tensor diag ( 1,-1,-1--1). The relation is equivalent to the inequality (x) + (y) > or= 2 Re () where the first member is the sum of the mean values of the operator T in the states x and y respectively. ( Gennaro Franco, Giuseppe Marino, Possible causal relationship extensions and properties of related causal isomorphisms, Linear and nonlinear analysis, january 2020)