 There are two aspects of the measure of existence of a world. First, it quantifies the ability of the world to interfere with other worlds in a gedanken experiment, as expounded at the end of this section. Second, the measure of existence is the basis for introducing an illusion of probability in the MWI as described in the next chapter. The measure of existence is the parallel of the probability measure discussed in Everett 1957 and pictorially described in Lockwood 1989 (p. 230).