The over-arching theme of organics versus machines shaped all the story choices
By the end of Mass Effect, players knew that the reapers and Geth were synthetic creatures (robot types) and somehow had inkling that the sequel of Mass Effect would have a confrontation of organics (Human, Asari, Quarians) and Synthetics. Casey Hudson the producer of the Mass Effect series envisioned a world, where the organics would try to have a better understanding of who the synthetics were and how they came to be. Case in point EDI an AI (Artificial Intelligence) aboard the Normandy goes from a simple AI to an unshackled fully aware AI.













