Hartle and Gell-Mann already establish that the cognitive functioning of the IGUSes (Information Gathering and Utilizing Systems) presupposes conditions of stability and the mutual exclusion of sequences of events. For an IGUS observer, whether natural or artificial, only one branch of universe can be endowed with a real existence; if this conclusion does not exclude in any way the possibility of other branches of the universe, it forbids any access to them to a given observer: to use the quite mysterious but synthetic expression of Gell-Mann, "On every branch, only this branch is preserved." The very presence of a community of observers, even reduced to two IGUESes, was thus proof of the existence of a reality.