Purity of form is a concept first pioneered by the xel'naga, and is defined as the ability for a race to have a great psionic potential. Standing in contrast to purity of essence, the xel'naga applied this concept when choosing a species to uplift to merge with another that possessed purity of essence, in order to rejuvenate their race in their cycle of reproduction. The race with purity of form required the ability to house the essence of a xel'naga.