The I was a sentient number.
A cult which believed God was an impossibly long number accidentally captured the I on Fleming's Island off the coast of Ireland in 1981. This attracted the I Predator to the area and it killed all the cultists. When the TARDIS landed on the island in December 1981, the Fifth Doctor and Adric freed the I and trapped the I Predator in a feedback loop generated by a calculator. (AUDIO: Iterations of I)