Prophecy that by its very nature cannot be denied; a major philosophical and psychological force during the melange-dominated age of the Atreides dynasty. Originating in the Terran concept of fate or predestination and continuing through Zensunni and Bene Gesserit philosophies, ijaz became a self-fulfilling prophecy itself. It was made undeniable and immutable in part by the collective belief in its reality and potency. Moreover, the visions of the future provided by melange and by the Ceremony of the Seed offered partial and clouded extrapolations, reinforcing the belief that there were in fact inviolate prophecies and unalterable future paths. The devastating wounds of ijaz, opened time and time again by the melange, finally destroyed the Atreides and brought about, indirectly, the Starvation and the Scattering. With the breeding of Siona Atreides by Leto II and Duncan-the-Last by the Bene Tleilax, Leto II hoped to introduce chance to sentient beings, bringing their consciousnesses into harmony with the nature of existence. Leto planned to resolve a debate between fate and free will that had been belabored for over 20,000 years. Through Leto II's death and the Siona-Duncan union, some think actual freedom was introduced for the first time, and the destruction of ijaz brought humanity into a true harmony with the chaos of the universe.

from The Dune Encyclopedia