After working on a 'Difference Engine', to mechanise the particular numerical method used in the construction of mathematical tables, Babbage had conceived (by 1837) of an Analytical Engine, whose essential property was that of mechanising any mathematical operation.
It embodied the crucial idea of replacing the engineering of different machines for different tasks, by the office work of producing new instructions for the same machine.
Babbage did not have a theory like that of Computable Numbers, to argue for universality, and his attention was focussed upon operations using numbers in decimal notation.
Yet he did perceive that its mechanism could serve to effect operations upon symbols of any kind whatever, and in this and other ways the Analytical Engine came close in its conception to the Universal Turing Machine.
Babbage wanted a 'scanner', in effect, working on a stream of instructions, and putting them into operation.
He hit on the idea of coding the instructions on punched cards, such as then were used for the weaving of complicated patterns in brocade.
His plans also called for storing numbers in the form of positions of gear wheels. Each instruction card would cause an arithmetical operation such as 'subtract the number in location 5 from that in location 8, and put the result in location 16.'
This required machinery he called the 'mill' to do the arithmetical operations, but the crucial innovation of Babbage's plans did not lie in the efficient mechanisation of adding and multiplying.
It lay in his perception that it was the mechanisation of the organising or logical control of the arithmetic that mattered.
In particular, Babbage had the vital idea that it must be possible to move forwards or backwards among the stream of instruction cards, skipping or repeating, according to criteria which were to be tested by the machine itself in the course of its calculation.
This idea, that of 'conditional branching', was his most advanced.
It was equivalent to the freedom allowed to Turing machines, that of changing 'configuration' according to what was read on the tape, and it was this that made Babbage's planned machine a universal one, as he himself was well aware.