It was an idea that had arrived somewhat by chance.
On the American side, they had thought of storing instructions internally because it was the only way of supplying instructions quickly enough.
On the Turing side, he had simply taken over the single tape of the old Universal Turing Machine.
But neither of these reasons for adopting stored instructions said anything about the possibility of interfering with the instructions in the course of a computation.
On the American side, it was not pointed out as a feature of the new design until 1947.
Equally, the Universal Turing Machine, in its paper operation, was not designed to change the 'description number' that it worked upon.
It was designed to read, decode, and execute the instruction table stored upon its tape.
It would never change these instructions.
The Universal Turing Machine of 1936 was like the Babbage machine in the way that it would operate with a fixed stock of instructions.
(It differed in that that stock was stored in exactly the same medium as the working and the input and output.)
And so Alan Turing's own 'universality' argument showed that a Babbage-like machine was enough.
In principle there was nothing that could be achieved through modifying the instructions in the course of an operation that could not be achieved by a universal machine without this facility.
The faculty of program modification could only economise on instructions, and would not enlarge upon the theoretical scope of operations.
But that economy could, as Alan said, be 'Very powerful'.