 As later emphasized in Dorato (2017), information is best not understood as a primary notion. It must be defined physically in terms of something else; as such, it can play an important notion in “theories of principle” in the sense of Einstein (1919). In RQM, the information is defined relationally as relative information (in the sense of Shannon) that a physical system has about another system. Relative information is physical correlation between two systems (see Rovelli 1996), namely a measure of the difference between the possible number of states of the combined system and the product of the number of states of the two systems, due to the existence of physical constraints. Thus, we say that a variable \(O_A\) of a system has information about a variable \(A\) of another system if the values that \(A\) and \(O_A\) can take are correlated. In the spirit of Shannon, this is a very weak definition of information that has no mentalistic, semantic, or cognitive aspects. The strong informational perspective of the early work in RQM has influenced several later development of information theoretical approaches to the foundations of quantum theory (see below).