Because established science views the physical body description of the "gray" as hypothetically "scientific", the overriding theory of many ufologists is that these creatures are quite possibly "real". Of course, the physical attributes of the gray are only seen as "credible" by modern science because its attributes fit neatly into their theory of evolution. The "reasoning" goes something like this: The small, slender-bodied, big-headed creature "evolved" from other human-like beings somewhere in the Universe. The large head is attributed to its advanced intelligence. As the humanoid "evolved" and became smarter, the head, skull and brain cavity grew also. Because the creature was "evolving" into a more intelligent being, with apparent higher brain functions like "telekinesis" (moving physical objects by thought) and "telepathy" (communication by thought without spoken, audible words), the being no longer needed its mouth or limbs, so they became unnecessary, weak and frail. Evolution teaches that such creatures are possible (and even probable) because they look like the next evolutionary step above humans. Ufo beliefs expert, Martin S. Kottmeyer writes in The Encyclopedia of Extraterrestrial Encounters (2001); "The idea underlying the Grays did not and could not exist before the idea of evolution." (pg. 224)