Advaita Vedanta
Adi Shankara was smart and knew that people would wonder how he could say such an odd thing. He realized that many people would ask him, "If a person's soul is really one with Ultimate all along, then what makes a person feel so separate from Ultimate?" His answer to this was that we are ignorant of our real self being Ultimate because we see through a kind of filter—like looking through a dirty piece of glass—and he called this filter we look through, maya, which means "illusion" in Sanskrit.