From The Sourcebook of Magic, pg 351ff:
Alfred Korzybski (1933/1994) warned that the “is” of identity and the “is” of predication present two dangerous linguistic and semantic constructions that map false-to-fact conclusions. The first has to do with identity—how we identify a thing or what we identify with. The second has to do with attribution—how we project our “stuff” onto others and things without realizing it.
E-Prime empowers people to not fall into the “is” traps of language. E-Prime refers to English–primed of the “to be” verb family of passive verbs (is, am, are, was, were, be, being, been).
Writing, thinking, and speaking in E-Prime contributes to “consciousness of abstracting” so that as we make our maps of the world we recognize how they differ from the world. E-Prime enables us to think and speak with more clarity and precision by getting us to take first-person.