After the death of his wife in World War II, a computing expert and former colleague of Alan Turing, by the name of Charles Milton Porter was invited to Rapture by Andrew Ryan. Accepting the offer, he and his colleague Reed Wahl founded Rapture Central Computing and set to work on the most powerful mainframe computer that had ever existed. Inspired by the artist Auguste Rodin's sculpture, The Thinker, the machine was called the Rapture Operational Data Interpreter Network (R.O.D.I.N.) and later colloquially referred to as The Thinker. The machine was used to run the various infrastructural aspects of Rapture, including the automatic doors, the security systems, the pneumo postal service, the trains, the phone networks, the vending machines and the oxygen supplies, however tensions broke out when Wahl and Porter sought to use the machine for different purposes.
Wahl began to use the machines predictive capabilities for the purpose of financial gain, having the The Thinker predict the outcomes of sporting events and the stock market. Porter on the other hand was aware of the machine's ability to replicate human personalities should enough data about that human be fed into it, and tried to feed audio recordings of his dead wife Pearl into the machine in hopes of using its artificial intelligence to replicate her. Wahl disagreed with this, believing the humanisation of The Thinker would ruin it, he also began to splice himself heavily and due to his excessive genetic modification developed a mental illness which caused him to believe that The Thinker would eventually produce a predictive algorithm capable of predicting all events in the world.
To take complete control over The Thinker for his own ends Wahl had the machine fabricate a tape of Porter swearing his allegiance to Ryan's political enemy, Frank Fontaine, and sent it to Ryan. Knowing that Ryan's secret police would hunt him down and imprison him in Rapture's penal colony of Persephone he devised a plan to escape Rapture with The Thinker's program. Inside Persephone, Porter was turned into an alpha series Big Daddy known as Subject Sigma, as part of the experiments there, and remained dormant after Ryan fell and Sophia Lamb took over Rapture. After seven years of Wahl having control of The Thinker and attempting to develop his hypothetical predictive equation, Subject Sigma was awakened by genetic scientist Brigid Tenenbaum and The Thinker, who was impersonating Porter.
While Sigma had no memory of his true identity, The Thinker guided him through Rapture, while all the while Wahl made psychotic transmissions to him and tried to hold him back with a wave of minions and offensive machinery. Once Sigma finally reached The Thinker's core he learned of his both his and The Thinker's true identities, with The Thinker's replicated version of Porter telling him that he needed a familiar voice to guide him through Rapture. Sigma then fights and kills Wahl, who is holding the punchcard which allows Sigma to print out The Thinker's codes to take to the surface with him. The successfully obtains the code from The Thinker, and he and Brigid manage to return to the surface with them.