Alan Turing
loreprogramming
The mathematician whose 1936 paper defined what computation is, and what no machine can compute - the boundary every computer since has been built inside.
The paper was about the limits, not the possibilities: he described a machine in order to prove that some questions have no algorithmic answer. That the description turned out to be a blueprint is one of the field's larger accidents. His wartime cryptanalysis is better known publicly and less foundational technically, and the treatment he received afterwards is part of the record too.
Also known as: turing