Ada and the Analytical Engine
Her notes on the Analytical Engine imagined a machine that could compose music as readily as it could solve equations.
Ada Lovelace's notes on the Analytical Engine are remarkable for what they saw beyond arithmetic. The machine was designed to manipulate numbers, but Lovelace recognized that numbers could stand for other things: notes in a musical composition, symbols in a logical argument, patterns in a sequence. That insight opened the door to general-purpose computation.
Her work also included something close to a modern program. The famous example for computing Bernoulli numbers lays out a sequence of operations, including a loop, in enough detail that the engine could in principle carry it out. It is a reminder that the idea of programming arrived alongside the hardware, not after it.